Sir, best you consult a Robbie R-22 IPC to check where the engine and rotor tech sensors are mounted. The IPC will also tell you the part number of the two sensors. Next, from looking at where the pulses come from, you can calculate the pulses/sec, hence the frequency of pulses that corresponds to 100% indication. The wiring schematic in the IPC will show you how to connect it up.
Where do you wish to use this tacho? A heli or gyro?
Cheers, Francois