Looking great David!
I am not familiar with USD forks but can't you just take the fork caps off and remove the springs to let the forks drop to their maximum travel? That is easy on conventional forks to determine max travel. Of course, if the bike won't roll with the springs out because the tire is rubbing on the fender, you will want to make the internal limiter/bump stops longer.
With conventional forks the springs don't even need to be removed if you don't mind some oil dripping off the springs that are sticking out the top.
A bit messy that way though.