Crankshaft Big end and Main surface roundness tolloerance

When rebuilding a mostly OE engine

(my case an NA6 MX5)

what roundness tollerance would I be looking for to make sure the crank is still "round" on the big end

and on the mains

other worse

Measuring 2-4 different locations around the crank surface, how much variance should I be seeing?

This sort of information is always contained in the workshop manual which is why I recommend getting hold of this before starting your build. Particularly for a car as popular as the NA Miata, finding the manual should take about 30 seconds work on google. With this in mind the official Mazda limit for out of round is 0.05 mm or 0.0019". That may be fine for a road car but for a performance engine I'd be much more conservative than that. Personally I'd like to see no more than 0.02 mm out of round on a journal of that size.

You should also check for taper - careful inspection of the old shells can also tell you a lot about the crank and oiling.