Somewhere, I saw a picture of how to measure drop with a string. The basic method involved standing the gun vertical (unloaded of course) elevated off the floor a few inches on a couple books or blocks, tie a small weight (large nut or other weight) on a string, hold the string against the rib at the muzzles, adjust the angle of the gun until the string just bears against the rib at the breach end, and measure distance from string at comb and heel.
Sounds a bit difficult to me. I have a long straightedge that will do the job.
