I agree with Judee in the calculation process. That seems to work for me as well. For me and my ME level I use:
220-51x0.50, which gives me 84.50 HR at AT.
The trick is keeping it in the range! And what needs to be done to keep it in range varies widely from day to day, or even a span of a few hours. I always find one of the hardest things to manage with activity of any kind.