Microgrooves have an even number of grooves, so simply measuring the largest diameter of the slug is the procedure.

Blue Moon,
My advice would be to go with the .311" and the .310" since the Lee sizers are so inexpensive. I shoot a .309" in a couple of .30-06's but this Winchester 94 that I have likes the .310" minimum or I get poorer grouping. I use one of those dial calipers to measure with. It's quicker.

I figure a bullet's only too big if the loaded round's too tight a fit in the chamber.