Maffetone 180-formula and HRM-based treadmill training

Since I've started using the Maffetone Method for training, I've run exclusively on a treadmill; specifically, a Life Fitness T3-5. I've had it in my house for five years, and I really like it. It works very well with the Maffetone Method, since my pace can be controlled via heart rate.

I've recently changed my routine just a bit, for three reasons:

1. I haven't been following Maffetone's 180-formula, and I'd like to get closer to it.
2. The way the Life Fitness heart rate control works puts me even farther away from the 180-formula. It allows me to go 3 or 4 beats per minute faster than my set target without slowing me down; this happens often near the end of longer runs.
3. Although when I started I was skeptical of the practicality of 180-formula for me personally, the improvements in my maximum aerobic function surely allow me to run at a much lower heart rate than when I started.

In short, I've targeted a heart rate of 138 beats per minute, compared with the 180-formula's 127; the Life Fitness treadmill bumps me up even more to about 141 or 142; and my improved fitness will now allow me to run at a lower heart rate.

Between 12 Feb and 22 March, my pace at 138 beats per minute dropped by about 58 seconds per kilometer (just over 1 1/2 minutes per mile). The main reason I didn't use the 180-formula to begin with was that my pace at 127 beats per minute would have been very slow; almost a walk. It probably would have been a walk at the end of an hour, although I admit that I didn't actually try it.

Rather than abruptly change from 138 bpm to 127 bpm, I'm doing it gradually. Before I was keeping my heart rate at 138, but bumping up my speed once I had been able to maintain a given pace for two days. I'll continue to do that; but I'll also drop my target heart rate by 1 bpm as well.

This has slowed my pace progress a little; I'd been speeding up every two days like clockwork for more than two weeks. My last two up-ticks in pace, along with a down-tick in heart rate, have both required more than two days to maintain. I'm currently at 136 beats per minute. I don't know if I'll ever get all the way down to 127. Before I get there, I'll probably have moved from Maffetone's category B ("inconsistent [training] or are just getting back into training") to category C ("training consistently"), which will move my target up 5 beats per minute.