When I set up cleats for float, I ballpark 'em in the middle and go for a ride. I pedal hard, maybe do some medium effort intervals or something, and stop with my feet level - then I twist one foot either in or out to see if I have play. If I do, great. I have adequate float. I repeat the process for both feet, and both directions. You have to be conscious to keep your feet exactly where they are falling naturally in the pedal stroke.

This has always worked for me, but of course there are tons of variables to cleat position. Best to talk to a pro.