Part # 20045A31 will give you a 3" dia. and a 20045A35 will give you a 8" dia. Any number in between those part numbers will give you a different size. They are about $14.00 each
I use a digital caliper and an excel spread sheet. Granted my method forces you to disassemble the engine to measure rods and ports; but every time I take in a new engine it gets a once over any way. I like this method because the whole engine ends up documented.
Never used a degree wheel that bolts to the crank.