I have found this sizing tool to be right on for me:
http://www.coloradocyclist.com/bikefit/
Measure yourself twice and then input the numbers in there and you will get your "ideal" sizing.
Clearance over the top tube is important but you also need to consider the length of the top tube as you don't want to be riding all cramped. If it helps I am 6 2" with a 34" inseam and ride a 19" CT. I can also ride 18" or 20" (also CT) but 18s feel too cramped and 20s usually give marginal crotch clearance. Besides this online calculator you can go to a bike shop and hop on to a few bikes and see what size fits right.