I've never seen a dyno that could measure cfms pulling through a carb. Blue's probably right about most people erring on the larger side than smaller. I've heard it said that a motor can only use what it can use no matter what size you put on. Every motor is built different so I'd say there is no one right and one wrong one. Just whatever works best. It would be great to be able to test several different ones to see what works best but that's pretty hard to do. In the end I guees you just have to trust someone that has supposedly done that, like the Pontiac engineers that decided a 750cfm was the right all round size for most of their applications.