Just measure from the tail shaft of the gearbox(oil seal) to the flange on the diff your prop guys will work out the rest but tell them that
Normally you would have 50% in the box 50% out
To much spline in on full suspension compression the prop will smash in to the box and damage the Gear box or the prop or both
Not enough spline in the gear box on full suspension drop the prop will fall out and damage the prop or could and at worse turn the car over and if you don’t turn over your rear tiers will be coated with gearbox oil either way it will end up in a nasty accident