Haven't tried RG but I'll give him a go. Charlie got a box from him.........not overly impressed.
I can't see how guess work can come into play when we are shooting for two thou / .05mm clearance.
That is incredibly close to no clearance. If we get no clearance , we're shot.
A known absolutely flat and parallel shim plate, with bearings fully seated in the front pockets, is the only way to be really sure of an accurate measure imo.
Straight edges across the rear of the tranny will give an estimate, but unless the rear bearing is perfectly in line with the front bearing, variations will occur.
Without doubt tho', there will be people who have developed a system that has been proven time and again, without needing a shim plate. That isn't me.
Unless I source a 'known absolutely flat and parallel shim plate' my best bet will be to be absolutely sure of the thickness of the 'not parallel' donor rear cover plate (in my possession) at one point for each bearing hole, and be done with it.
I have a reputation to......improve on.
Lord of the Bings, kangaroo farmer