I've seen a figure for the flanges on both layshaft bushes, and 0.093" comes to mind but I think each end is different to each other.
It's there on the nets somewhere.
Otherwise run a straight-edge across the faces and measure the gap from the interface to the 1st gear before measuring from the interface on the inner cover (biscuit/sammidge
plate) to the bush flange.
On the ones I have, from memory this is almost flush, but whatever- do the maths (all of) and you should work it out fairly easy. If the bush flange is too thick is in my opinion better than adding shims as the flange can be fettled to suit (shims can be messy)