Actually, the factory shim thickness is .030". But as Jeff said, it could vary. It's unlikely though since these rears are assembled on an assembly line. You can be pretty certain that those people aren't using pinion depth gauges and doing pattern checks. I recently did my gears and all the old shims were correct, but I was going from Ford gears to Ford gears. The left carrier shim is a standard size, too. .265" You will have to tweak that to get the back lash you want. You'll need a dial indicator.
To get the pinion depth you have to take the pinion head size and subract that from master housing dimension (4.415").
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351W 89 Mustang GT Convertible