Several ways can be found here:
The way I like from those is to put your cue tip at the ghost ball and sweep out the angle from the path of the object ball to the path of the cue ball. The number of inches your bumper moves is the number of degrees in the cut angle. (Technically, you have to measure the length of the arc and not the straight line distance between the two positions of the bumper, but for angles up to 30 or so, those are nearly the same.)
There's a diagram on Dave’s page.
