I have a zoom lens that only has a handful of focal lengths marked on the zoom ring. I want to make some intermediate marks, but I don't know the math required. I do have the approximate angles of the factory-marked focal lengths. The lens zooms from 70mm to 180mm over 110 degrees of rotation. If 70mm sits at 0 degrees, 90mm is 40 degrees, 110mm is 70 degrees, 135mm is 88.75 degrees, and 180mm is 110 degrees. How can I calculate the angle for, say, 85mm or 150mm?