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.
4 radians = 4 X 57.296 = 229.184 + 17 degrees = 246 degrees.
So, apparently the Zodiac Killer had some concept of radians and the value of Magnetic North, unless we are assuming he was just lucky in his placement of the bold circle on the diagram. His further reference to SFPD (San Francisco Police Department) next to this circle, suggests this is where we should be centering our focus, bearing in mind his disdain for law enforcement.
The Zodiac Killer wrote "The Mt Diablo code concerns radians + inches along the radians", so let us start with complete radians. We know from above calculations that the value of 4 radians (229 degrees) is significant, so we shall place a compass rose over Mount Diablo and see where this takes us, in respect of San Francisco.
Was the Zodiac Killer intending to ratchet up the fear by exploding bombs (or the threat of bombs) in the vicinity of the airport? This seems highly unlikely, so we shall take a further look at the Button letter mailed on June 26th 1970, where the Zodiac Killer stated "The Map coupled with this code will tell you where the bomb is set. You have until next Fall to dig it up" and enter something alongside the code.
SEE PART TWO - http://www.zodiacciphers.com/zodiac-news/sfi-airport-part-two-hunters-point