The Zodiac Killer knew the prominence of Mount Diablo from an elevation standpoint, but very likely knew some of the history and purpose of Mount Diablo - and why he chose this mountain as his "initial point" for the code of radians and inches to be measured from.
It is true that only a tiny proportion of a population would understand what a radian is, but the conclusion drawn from the Zodiac Killer's use of radians as an angular measurement, that he must have been familiar with them through his profession or hobby, is not necessarily the case. He could easily have read a book, magazine, or newspaper article in the preceding hours, days, weeks or months to mailing the Button letter. While most people will not be knowledgeable to the concept of radians, it takes very little time to familiarize oneself with them. Therefore, the idea of radians being a requirement to his daily life through his work (past or present) is not necessarily the case. It may have been a topic he had recently read and so decided to utilize the measurement of radians in his Mount Diablo code. Despite highlighting this point, it is clear that the Zodiac Killer through his communications with the newspapers, did exhibit good "mental mapping" skills in relation to the environment around him. His compass skills were correct in all instances, whether by use of a map or through an extensive knowledge of the locations he murdered in.
When you factor in the cryptic directions given in the Pines and Monticello cards, it becomes increasingly likely that the Zodiac Killer had a keen interest in maps, the surroundings in which he lived and worked, along with the hunting grounds he chose for murder. It may suggest that he methodically planned out these attacks well in advance, so as to facilitate the best possible chance of escape. Taking this all into consideration, it now becomes a much more reasonable argument to suggest that the Zodiac Killer may have had prior and extensive knowledge of radians, rather than just a passing whim prior to his mailing of the Button letter on June 26th 1970.