well you may not want to hear this but the best thing would be to experiement and make your own solution. Buy a tachometer. You sound like you know the basic elements: motor, pinion, TX setting, maybe a governor.
This is your brain on helicopters!
Whiplash Gasser w/ BD-3SX
Trex 700N FBL Gasser conversion w/ BD-3SX, Trex 550E FBL BD-3SX, 450 FBL, Gaui X2, mCPx