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!
Trex 700L, Trex 700N DFC HB gasser,Trex 700N V2 HB gasser, Trex 550E FBL all with BD3SX