The Button Letter, stated ‘The Mt Diablo code concerns radians + inches along the radians.
This suggests WHOLE radians.
The Little List Letter contained a black circle around the crosshair.
The Button Letter Phillips Map talked of setting magnetic north.
The Little List Letter black circle is centered exactly 4 radians and 17 degrees (MagN was slightly under 17 degrees in 1970)
One radian = 57.296 degrees, 4 radians is 229 degrees and 4 radians + 17 degrees is 246 degrees.

The Zodiac stated "’the map coupled with this code will tell you where the bomb is set’ on the Button Letter.
In other words one plus the other will reveal the location.
Since the bombing references were centered over San Francisco that’s where we shall look.
So lets look at the code first, it is 32 symbols in length. So is San Francisco International Airport. This was the destination of Paul Stine’s final fare.

San Francisco International Airport is 4 radians or 229 degrees from Mt Diablo.

But the code is to be coupled with the map of Magnetic North of 17 degrees.
229 +17 = 246 degrees, the exact position of the black circle on the Little List Letter.

246 degrees passes over the Bayview-Hunters Point region of San Francisco and the Hunters-Point Naval Shipyard.

The Long Beach Naval Shipyard, which closed in 1997, was located at Terminal Island between the city of Long Beach and the San Pedro district of Los Angeles and approximately 23 miles south of the Los Angeles International Airport. It was situated 1 hr 20 minutes drive west of Riverside. The Mare Island Naval Shipyard is located in Vallejo, the hot zone highlighted by Kim Rossmo, when using the art of geographical profiling to zoom in on a possible home location of the Zodiac Killer. And finally the Hunters-Point Naval Shipyard, San Francisco, located on 638 acres in the southeast corner of the city.
Three shipyards, the naval connection and Hunters Point, I wonder if any irony was involved.

Finally, Let us expand upon the ‘Map coupled with this code will tell you where the bomb is set’ to better explain things, using this phrase ‘The Phillips 66 Map coupled with this code will tell you where the bomb is set’.
In the first instance the code of San Francisco International Airport or 229 degrees was coupled with the map set at Magnetic North of 17 degrees to equal 246 degrees. The Mount Diablo code was written on the 180 degree axis, so we will couple this 180 degrees with the map or more specifically the Phillips 66 Map, to give us 180 + 66 = 246 degrees, the exact position of the bold circle he provided us in the Little List Letter and exactly the same as 4 radians and 17 degrees. Then we shall take the number 6 he provided on the 180 degree axis and once again couple this with the Phillips 66 Map to give us 666, the mark of the Devil or Diablo. The conventional view is that the peak derives its name from the 1805 escape of several Chupcan Native Americans from the Spanish in a nearby willow thicket. The natives seemed to disappear, and the Spanish soldiers thus gave the area the name "Monte del Diablo", meaning "thicket of the devil."
Diablo was placed over the 180 and 6 axis and on both occasions coupled with the 66 from the Phillips 66 Road Map gave us something significant, but whether or not this was his true intention is still unclear and may never be resolved.