The formula is reasonable for populations not individuals. If you want to know your max you'll need to measure it when you're going all out. Find a hill you can climb steadily at a hard pace for a few minutes and then sprint all out until you can't go any more. Your peak HR should come just after you stop.

Alternatively, it's probably better to structure your intervals around your threshold HR. There is a sticky at the top of this subforum (http://www.bikeforums.net/showthread...threshold-test) for measuring your threshold HR.