[LONGHAIR]

It's simple geometry.

Measure the distance from the center if the front bushing of the trailing arm to the center of the axle. Then measure the distance from the axle to the rubber bump-stop. Then you can make a simple tool to measure it and see it with you own eyes.

Cut to straight sticks (2x4s or whatever you have) that are a few inches longer that the previous trailing arm measurement. Line them up, one on top of the other and drive a nail thru them so that it is the same distance from the end as the measurement (A). They will then open forming a V from that point. Now place them on the edge of a work bench/table, line the bottom one up with the edge of the table. Open the other one to the distance of the bump-stop(B). Now the measurement from the opened piece to the edge of the table is how far your axle will move(C).
Here is a picture to explain it a bit better..........