The numerous suggested bomb threats were all mailed to the San Francisco Chronicle, so it was evident that his threats were focused on San Francisco itself - which is why the Mount Diablo map included San Francisco and Vicinity rather than just the area around Mount Diablo. The 'Little List' letter was replete with all the information required to crack his map code.
In the 'Button Map and Code' letter the author gave us the "map coupled with this code will tell you where the bomb is set," and just one month later, clearly told us that the code "concerns Radians & # inches along the radians" Therefore, the code will at least contain the words "radians" and "inches" in the answer - and that may very well be the case. In fact, it tells us exactly where the intended bomb was to be set, within the design of the crosshairs with the bold black circle. With the threats geared towards schoolchildren and police it is apparent that these are the likely targets. The reference to SFPD = 0 being incorporated within the crosshairs, along with an accentuated black, bold zero suggesting a location, should be the biggest clue of all. His intended target was the San Francisco Police Department - the location of which can be found within this bold circle.
The Zodiac Killer referenced "radians" and "inches" (not parts of radians and inches), so with probably just the use of a ruler back in 1970 the Zodiac Killer would measure his intended target this way. Therefore, if he is one eighth of an inch off in his measurements we can forgive him this textual indiscretion. Fortunately, we have Google maps, so the task is much easier.
With an intended target in San Francisco, there is only one answer possible to the "radians" and "inches" conundrum, and it's governed by geography. 1 radian = 57.3 degrees, 2 radians = 114.6 degrees, 3 radians = 171.9 degrees, 4 radians = 229.2 degrees and 5 radians = 286.5 degrees. This is detailed crudely on the map below.
It is apparent that Zodiac wasn't referring to 5 radians, as a line subtended from Mount Diablo at this angle would pass north of San Francisco. The same applies to 3 radians which runs parallel to San Francisco. Therefore, the answer has to be 4 radians or 229.2 degrees. The lines drawn below are only approximations, so are not pinpoint. 4 radians or 229.2 degrees passes over San Francisco somewhere in the vicinity of the airport - but this isn't the answer. If we add back the magnetic north value of 17 degrees in 1970, it equals 246.2 degrees. After all, Zodiac stated on the map, it was "to be set to Mag.N."
How do we know 246.2 degrees is the correct angle to be subtended from Mount Diablo? The answer is simple - Zodiac told us so. Place the crosshairs above over a compass rose. A line subtended from the center of the compass rose passes exactly through the center of the bold black circle. In other words, the bold black circle is positioned at 246.2 degrees around the crosshairs. This is 4 radians + 17 degrees (magnetic north). Now all we have to do is find the inches subtended along the 246.2 degree line. This is even easier.
It is approximately 26.5 miles to the eastern edge of San Francisco from Mount Diablo (using the angle of 246.2 degrees).
4 inches would equal 25.6 miles, therefore, 4 inches subtended from Mount Diablo would land in the sea. Unless the Zodiac Killer had a submarine to bury his bomb this is not very likely.
It is approximately 34 miles to the western edge of San Francisco from Mount Diablo (using the angle of 246.2 degrees).
6 inches would equal 38.4 miles and again would land in the sea.
Therefore, the answer has to be five inches along the 4th radian (plus magnetic north of 17 degrees). If this lands over the San Francisco Police Department we are in business. But first, we shall fill in the '32 Symbol' code using the words or clue given by the Zodiac Killer in the July 26th 1970 'Little List' letter.
The section below is almost the complete solution to the '32 Symbol' cipher. Note how the Zodiac Killer inserts a hashtag style character between the "&" and "inches", with an arrow pointing between these two pieces of text. He is telling us to insert something here. We know from the above calculation it must be five inches, thereby completing the phrase:
"P.S The Mt.Diablo code concerns Radians & five inches along the radians"
There are only three repeating characters in the code below, the triangle, the letter C and the letter O. With the inserted solution, these three characters are now represented by R, A, N, N, E and N.
The last thing we have to do is place the large crosshairs with the black circle over the map of Mount Diablo, San Francisco and Vicinity. The location where the intended bomb was to be set should be 246.2 degrees and 5 inches subtended along a line drawn from Mount Diablo. It should also land over something to do with the San Francisco Police Department. Bearing in mind this was 1970 and the Zodiac Killer was using a ruler over a map, one eighth of an inch or so would be an acceptable margin of error, based on a map scale of 6.4 miles to the inch. The center and style of the black bold circle may be representative of a designer who intended the target to fall within this circle.
There was no room on the map below, but we can see that if the superimposed crosshairs were expanded over the city of San Francisco, the center of the bold black circle subtended at 246.2 degrees for 5 inches would fall over the San Francisco Police Department at Ingleside Station.
Ingleside Police Station is 31.06 miles from Mount Diablo. There are 6.4 miles to the inch. So Ingleside is 4.85 inches along the subtended angle of 246.2 degrees. Allowing for the leeway of crude measurement, this is 97% accurate in respect to 5 inches over the distance involved. This is why the Zodiac Killer placed SFPD next to the bold circle - it represented zero in mocking fashion, but more crucially, it represented the location of where the bomb was intended to be set - Ingleside Station or somewhere close by.
Footnote: What level of understanding the Zodiac Killer had regarding the concept of radians is unknown, but they are a counterclockwise value around the circumference of a circle. He placed his crosshairs over Mount Diablo, to which he added a sort of clock face, replacing the number twelve with a zero, stating it was "to be set to Mag.N". It can be shown that 4 radians and Magnetic North will fall within the bold, black circle he placed on his 'Little List' crosshairs. In addition, it can be shown that traveling 2 radians counterclockwise from True North will again fall within the bold, black circle. The use of the clock face and the directional north marker set at zero, may indicate that Zodiac intended this as his starting point for any calculations. The mere fact that the above measurements, either clockwise or counterclockwise, using whole radians, both fall inside the black circle, would seem to suggest that the 246 degree angle subtended from Mount Diablo over San Francisco is where we should be looking.