The shortest distance between two points is a straight line, not angled.
As the rearend assembly "drops" lower or hangs on the leaf springs, this pulls the slip yoke out of the trans. As the rear gets "loaded" (springs get fully compressed) that pushes the yoke into the trans...

Fact are:
If the slip yoke hangs-out of the trans too far, it wobbles, vibrates and beats-out the rear bushing in the trans. (Driveshaft is too short)
If the yoke is too far in, it hits the trans tail housing / output shaft, under full spring compression and bends or breaks parts. (Driveshaft is too long)
You need at least 1/2 inch of slip yoke forward movement, in order to clear the rearend yoke's "saddle face", to get the u-joint "cross" into the knuckle's semi-circles.

This photo shows a slip-yoke that is "pulled-out" when the car is jacked-up, letting the rearend hang free.
When I go to measure for a driveshaft, I have the rearend installed, the vehicle's weight on it, (at ride height) install the slip yoke all the way into the trans, pull it back out by 1 inch and take the measurement.
DD

Thank You Doc for the tutorial, I appreciate it!
NOW I get it!