The out of round you have measured is common. I have found the outer bore will distort between 0.010"- 0.015" subject to the pounding (high rpm,etc) the engine has taken.
Richard
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Thats a great point to show there, thank you for doing that. (Well illustrated). ALL of these vintage bike alloy case engines distort to some degree over time. I find it amazing that few people are willing to acknowledge that after 60-70 years that these things distort.
I have not extensively surveyed early BSAs but I can attest that multiple Unit twins certainly do distort and a tool & die maker friend used to blue print cases for people and found a lot of BSA preunit singles were badly out of wack or even manufactured out of tolerance. I have a file somewhere where I documented much of his procedure and need to re-locate it but the last one he did before he died was the Main bushing bore WAS distorted like you show but ALSO off center to the other bearing bore, and the cam bushes were also out of line but in a different direction. When you bolted up the cases it locked everything up. Took quite a bit of work to correct.
Not all doom and gloom however, its fixable but every engine should be checked.
As to using a Mic, calipers, snap gauges or anything I am sure there is probably tutorials online these days, perhaps some Youtube videos? But the best thing is find a skilled and seasoned machinist and bribe them to give you a tutorial. Then have them measure stuff and you do the same until you get repeatable and accurate measurements. People who have not done this in a while should also do a refresher as there is a goldilocks touch to it.