You calculate FTP from an equation? Maybe that's the problem. I can't think of anything simpler to measure than average power for an hour TT. Ok, its no fun but that's another problem. But I have never been able to accurately determine my LTHR through a field test. And lactate threshold isn't "real"....even doing it the "scientific" way with a lactate tester it's open to interpretation. You graph the results and look for where the slope changes.

Power is much better for short anaerobic efforts since HR lags output by quite a bit. If you do an interval at exactly 300 watts your HR will rise over the course of the interval- quickly at first then more slowly through the interval. If you pace it by HR you'll be going much harder at the beginning and will slow down through the interval. Now maybe that's what you want to do, but wouldn't it be better for your measuring instrument to be showing you that?