This calculation was created by Dr. Phil Maffetone and has been tried and tested by numerous athletes, including Mark Allen.  It’s a good calculation and has been shown to quite accurate in most cases for determining your Maximum Aerobic HR: a.k.a. the point below which you should keep much of your Ironman training, and most of your winter training.  We started using this limit in the winter months of 06/07 for all of our winter training.  This meant walking up hills until our aerobic fitness caught up with the rest of our system.  Embarrassing at times for sure.  Come race season 2008, more than year later, the differences were unreal.  The one downside to this approach is you have to be patient and you have to train a lot.  As a minimum, 4 hours per week of running (on top of cycling, swimming etc.)would be a good volume to which you would see some major improvement.