I measure it by eye actually, but it's not hard to do. Try this diagram I just quickly did up in Paint:
Once the pad is inside out, you'll see the 4 stock damping holes, which you can use as reference. I use a permanent marker to mark the points. You can use a ruler, but I've done it so many times I can just do it without now
So mark points in the center from edge to edge, equidistant from each stock hole, then repeat between those points:
Next, you'll do two rows, each closer to the edges, in between the middle holes like so:
Once they are all plotted out, you can begin cutting or you can trace out the hole plan first. The middle holes should be bigger, and the outer ones smaller, like this:
Just repeat all the way around and you should be good! For your first time, definitely trace the holes out before cutting.
Here is a pic of a pair done so you get a better idea:
Edited by modulor - 8/29/13 at 5:35pm