ditto, I've always been told "the formula" is not accurate at all. I too have done sub max tests.... essentially you warm up for 20 minutes, and then ride at the consistent maximum that you can ride in for the next 20 minutes at a fairly high cadence( 90-100), take an average of your highest HR and then calculate:
http://lwcoaching.com/?p=138
http://lwcoaching.com/trainingplans/levelCalcs.htm