Very good info from everyone. Don'y get worried over this AT ALL.
I set the rear axle on jack stands, and the front wheels on blocks of wood. Push on the trunk to get the suspension to settle. Put the trans yoke all the way in, measure from center of UJ cap to center of UJ cap, the deduct an inch as James mentioned. My son's shaft is 51.25" with an 8.5 and 200-4R. But if you have adjustable uppers and lowers, then you can make up 1/4-3/8" in rear adjustment easily.
Don't go off from anyone elses measurement. Measure for your self. And being off by a 1/2" in either direction isn't any issue as long as your ride height doesn't change by 2 or 3".