Shortening driveshaft - how to measure?

Making progress on my A500 tranny swap.

As far as having my driveshaft shortened. I was just going to measure center of front u-joint to center of rear u-joint.
My question I guess is about measuring. I was planning on sliding my new slip yoke into the tranny till it stops, and then pull it back , what, 1" ? Then measure from the center of the new yoke to the center of the yoke at the diff.

Sound about right?