I am a big fan of Occam's Razor -- the simplest explanation is usually the best.
From the Gordo Byrn school (as influenced by Molina, Allen, Lydiard, Hellemans,...):
"My preferred evaluation method is to apply subjective perception. Start training at an easy pace and slowly increase the intensity of exercise. AeT is determined by noting the heart rate where you feel the first deepening of your breath. This deepening is caused by the body increasing its demand for oxygen to metabolize an increase in blood lactate.
"Many coaches and athletes prefer a more scientific approach to AeT determination. My view is that this takes away from one of the key benefits of AeT training, that being, having our athletes dial into their bodies. A flexible approach to determination is useful because AeT will often move 5-10 bpm based on an athlete’s daily recovery situation."
You know what? I've done "lab tests" and the above method works and is more accurate. Lydiard defined his "Maximum Steady State" as a pace that leaves you "pleasantly tired" after a long workout, but sufficiently easy that you can go out again the next day and do the same workout.
Lydiard's protocol is to run 45 minutes away from your house at what you think is the right pace; turn around and run back with an even split. Was it a little hard to hold pace? Back off a little and try it again a couple days later. Was the return trip pretty easy? Try it a little harder the very next day. [you can do the same thing on the bike, but ride 90 min out and 90 min back].
It is very simple to find the right pace if you just dial into that zone that is harder than easy, but easier than hard -- tempered by the fact that you have to be able to do it day after day. If you're going too hard, you'll know it after a few days.
Some days, my zone is 158 on the run; other days it tops out at 152. On the bike, I'm weak right now and 148 is a little too hard. After a few weeks, you'll just "know" where AeT is at all times.