Welcome to the job dodad70!
Here's the most accurate way I know to do this. Do just what you did only turn the wheel 20 times while counting the driveshaft revolutions, then divide your driveshaft number by 10
(this is for a non-posi rear, only jack up one side)
For a posi rear, spin the wheel 10 times (both sides jacked up) and divide by 10.
Sounds complicated but it really isn't. I use a sheet of paper divided into 2 columns and just make "tick" marks until I get to 20 turns.
