Slip joint-how much is enough?

[SunSoaked]

My front shaft has 5/8th of "free play or travel". At first, I thought this wasn't enough and have been contemplating getting it shortened. However, some observations of the front suspension tonight bring up a few things.
1. Spring compression causes the differential to move rearward-causing the shaft length to decrease. I believe the amount of movement is very small based on two observations:
1- The axle is positively retained by the spring mounting pad with u-bolts and a pin and, the front end of the spring is attached to the mounting bracket at the spring eye. It can only move by the spring flexing and causing the spring to lenghten "slightly".
2- The front sway bar is mounted to the spring mounting plate on the axle and also to the frame. This makes it impossible for the axle to move backwards except for, once again, the "slight" movement created by the arc created by the vertical movement.
I've only got about 2-3 inches if travel before it's against the bump stops. I'm guessing this amount of limited travel vertically may not translate to more than about 1/2 inch of rearward travel.
Does any of this make sense?
If there is an easy way to check it under max compression it would be easy but, how do you compress 3/4 leaf springs? I guess the next step is to take measurements from the mounting points and draw it out on some poster board.