This is the answer to the Mount Diablo Map and Code in eleven simple steps. The entire answer was supplied by the Little List letter (7.26.1970).
#1. The Button letter states "The Map coupled with this code will tell you where the bomb is set". This tells you one simple thing, that neither the map nor the code will give you the answer. You need the two elements of map and code to identify where the bomb is set. Therefore, any solution to the code which claims to pinpoint the bomb location (or where the Zodiac Killer had identified as a location) will be wrong.
#2. The following Little List letter states "P.S. The Mt. Diablo code concerns Radians + # inches along the radians". These are the two components to the solution - radians and inches. We know that the map coupled with the code will tell you where the bomb is set, so obviously one will contain the radian solution and one will contain the inches solution. Together they will identify "where the bomb is set".
#3. There are two crosshairs on the Little List letter. One has "P.S. The Mt. Diablo code concerns Radians + # inches along the radians" written over it, while the second has a bold, black circle around the circumference with SFPD=0 next to it. One gives the radian value, while the other gives the inches value. It really isn't that difficult to work out which contains each.
#4. The crosshairs with a hashtag before inches is telling you to insert a number here. The hashtag symbol represents a number sign. Look on Wikipedia and it explains it perfectly. There is an arrow beneath the hashtag beckoning you to insert a number value before inches.
#5. The other crosshairs have a bold, black circle around the circumference with SFPD=0 next to it. If you place this over a compass rose, the center of the circle falls over 246 degrees, which happens to be 4 radians and magnetic north. Remember, Zodiac stated on the Mt. Diablo map "is to be set to Mag. N." Magnetic north was 17 degrees in 1970. A radian is 57.3 degrees. So, 57.3 X 4 + 17 = 229.2 + 17 = 246.2 degrees. The answer to the map was therefore 4 radians and magnetic north.
#6. Next to the bold, black circle was SFPD (San Francisco Police Department), so it's pretty obvious the target was a police station in San Francisco. Virtually every other threat Zodiac made was directed toward San Francisco and this wasn't going to be any different. So let us now find the inches value.
#7. There is only one answer. If you travel 4 or 6 inches along a line subtended at 246 degrees from Mount Diablo (4 radians and Magnetic North), the location lands in the sea. Only 5 inches falls over the land mass of San Francisco. The same applies to the radian value. 3 radians and magnetic north, or 5 radians and magnetic north, doesn't land over San Francisco.
#8. Therefore, only one answer exists: 4 radians and magnetic north and 5 inches. The map coupled with the code will tell you where the bomb is set. So, 4 radians and magnetic north, coupled with 5 inches along that angular line is where the bomb is set (or claimed it was going to be set).
#9. The Zodiac likely used rudimentary measurement tools in 1970, however, to an overall accuracy of over 98%, 4 radians and magnetic north, coupled with 5 inches, lands over Ingleside Police Station. This police station was 4.03 radians and magnetic north from Mount Diablo. Ingleside Police Station was also 4.84 inches from Mount Diablo (using the map scale of 6.4 miles to the inch). This is the only police station anywhere close to these measurements and satisfies the SFPD=0 attribution. But can we be sure it's correct?
#10. The Zodiac Killer drew a diagram on the April 20th 1970 letter. It had a school bus traveling from north to south (with the sun rising in the east), with a steep embankment to the right, If you travel to the closest portion of road traveling north to south from Ingleside Police Station, you arrive at the Southern Freeway. This is a bus route for Thomas More Catholic School traveling north to south (with the sun to the east in the morning), with a steep embankment to the right. It is only 274 feet from Ingleside Police Station.
The Zodiac Killer by choosing this location, would have been able to blow up a school bus right next to a police station, thereby creating carnage under the noses of the police he so utterly taunted and reviled.
#11. Therefore, the solution to the map was 4 radians and magnetic north. The solution to the code may have been Radians and 5 inches along the radians (which is 32 characters in length). This is why he placed this portion of text on a separate line and capitalized the first Radians. However, Druzer, a contributor to both Zodiac forums, offered an improved answer for the code soluton of "estimate: four radians and five inches", which not only satisfied the 32 characters, but correlated three plaintext letters to the only three repeating ciphertext characters in the code and thereby provided a perfectly valid key. Either way, the bomb was to be set alongside Ingleside Police Station (SFPD) on the embankment of the Southern Freeway. See The Blast Estimate of Zodiac and The Blast Estimate of Zodiac (Part Two).
Footnote: Ordinarily radians would be measured counter-clockwise, but the Zodiac Killer gave us crosshairs superimposed over Mount Diablo with an arrow pointer set in the north facing position, "to be set to Mag. N". He allocated it a zero position, with a 3, 6 and 9 in the form of a clock face around its circumference. This was to instruct us to travel clockwise around the circumference of the crosshairs in this instance. Which is why it was no fluke that the bold, black circle fell exactly at 4 radians and Magnetic North.
For the full story: The Answer to the Mount Diablo Code
The Button letter stated "I have become very upset with the people of San Fran Bay Area. They have not complied with my wishes for them to wear some nice Zodiac buttons. I promised to punish them by anilating a full School Buss. But now school is out for the summer, so I punished them in another way. I shot a man sitting in a parked car with a .38. The Map coupled with this code will tell you where the bomb is set. You have untill next Fall to dig it up".
1963 June 4th-The Domingos/Edwards Murders
1966 October 30th-The Cheri Jo Bates Murder
1966 November 29th -The Confession Letter
1966 December- The Riverside Desktop Poem
1967 April 30th- Bates Had to Die Letter
1968 December 20th-The Lake Herman Murders
1969 July 4th-The Blue Rock Springs Attack
1969 July 31st- Vallejo Times-Herald Letter and
408 Cipher Page1 (solved)
1969 July 31st- Examiner Letter and
408 Cipher Page 2 (solved)
1969 July 31st - Chronicle Letter and
408 Cipher Page 3 (87% solved)
1969 July 31st-The Complete 408 Cipher
1969 August 4th- Debut of Zodiac Letter
1969 August 10th- Concerned Citizen Card
1969 September 27th- The Lake Berryessa Attack
1969 October 7th- The Good Citizen Letter
1969 October 11th-The Presidio Heights Murder
1969 October 13th-The Paul Stine Letters
1969 October 22nd-Call to Chat Show
1969 November 8th-The Dripping Pen Card and
340 Cipher (solved in 2020)
1969 November 9th-The Bus Bomb Letter
1969 November 19th- The Riddler Notes
1969 November 21st-The San Jose Code Letter
1969 November 28th-The Betsy Aardsma Murder
1969 December 7th-The Fairfield Letter
1969 December 10th- Forecast for Cancer
1969 December 11th- Forecast for Leo
1969 December 16th-The Fairfield Letter
1969 December 20th-The Melvin Belli Letter
1970 Feb 21st- The Hood and Garcia Murders
1970 March 22nd-The Modesto Attack
1970 April 15th-The Robert Salem Murder
1970 April 20th-"My Name is" Letter,
Cipher 3 and Bus Bomb Diagram
1970 April 28th-The Dragon Card
1970 June 26th-The Button Letter
Cipher 4 (solved in 2019) and Map
1970 July 4th-The Sleeping Bag Murders
1970 July 24th-The Kathleen Johns Letter
1970 July 26th-The Little List Letter
1970 Sept 6th-The Donna Lass Disappearance
1970 October 5th-13 Hole Postcard
1970 October 17th- "You Are Next" Postcard
1970 October 27th-The Halloween Card
1971 March 13th-The Los Angeles Letter
1971 March 22nd-The Pines Card
1971 Unknown- The 148 Character Cipher
1971 July 13th-The Monticello Card
1973 August 1st- The Albany Letter
1974 January 29th-The Exorcist Letter
1974 February 3rd-The SLA Letter
1974 May 8th-The Citizen Card
1974 July 8th-The Red Phantom Letter
1974 December 27th- Christmas Card
1975 November 3rd- The Belmont Letter
1978 April 24th-The 1978 Letter
1978 May 2nd- The Channel Nine Letter
1978 July 19th- The Scotch Tape Letter
1981 March 8th- The Atlanta Letter
1982 January (?) - The Santa Claus Card
1986 May 6th- The Freeway Letter
1987 October 28th- The 1987 Letter
1988 February 1st- The McDonald's Letter
1988 February 8th- The McDonald's Letter 
1990 September 25th- The Celebrity Cypher
1990 December-American Greetings Card
2001 January 10th- Happy New Year Card
Unknown DMV Letter (possibly November 1971)