I have a current tester and it pulls at 420MHz ~2,4A. The flat side of the "knob" is almost in this position: _. If I turn it like / or | can't reach 100%, gets sometimes super hot and stops around 76% max 88%, than "collapses", restarts at a lower frequence. I tried more than 420MHz i.a. 450, 470, 500MHz without success. I think it should work at 420 just fine, because it is definitely above the 200-300 "instability" threshold.
Mh, weird; so to be honest, mine pulls almost 3A as I said, so feel free to increase maybe to a target of 2.6A or 2.8A; I'm not entirely sure right now what position that would visually equal (depends on stick to stick as well). My stick is roughly between \ and |, if I look at the stick the way it sits in the hub (USB port to the bottom, vertically down pointing towards planet earth's centre). Starting at 350 to 400 sounds good, that's what I do too. You might have actually set it way too high, maybe try \ setting.