I would measure a/c directly to the dropout from the side of the crown. Then use that as the hypotenuse along with the rake to calculate the actual a/c as the other leg of the 90° triangle.

One advantage of this method is that you will get very close to the same answer regardless of whether you have the exact rake or not. Even if you get rake wrong by 10mm, it will only change the calculated a/c by 1mm.


This is the reason that measuring to a random point on the bottom of the crank also produces reasonably accurate results for the rake measure - the angles are too shallow to matter that much.