Checking differential side gear endplay. I am stumped!

[86Shelby]

I need some help checking/adjusting the endplay of the side gears in my A555 differential. I understand what the manual says and how to do it but to check one side of it the procedure seem somewhat haphazard.

1. While checking the side gear closest to the ring gear I should measure the distance from the raised portion of the ring gear to the flat machined surface. All fine and dandy.

2. Then I measure the distance from the machined surface on the other portion of the diff to the back of the side gear, with all the gears assembled in the diff. No shims on the side gears but shims on the pinion gears. That's fine but the side gear wobbles side to side a lot. How can I get a repeatable measurement from this? I'm stumped.