let's put it this way, THEORETICALLY:

At a CONSTANT mainblade rpm you also have a constant & proportional tail rpm

with the above assumption, you have a constant force on the frame of the heli going in the opposite direction [counter clockwise] of mainblade rotation (action reaction)

to neutralize this CCW force, the tail rotor needs to exert a constant force clockwise

therefore, you need a constant tail blade pitch AKA SUBTRIM on the rudder to make the tail push CW.

in reality the forces are not constant due to motor loading, friction, and battery depletion.

the rate gyro should only slow yaw movements that are too quick. the tail WILL still drift slowly and the gyro will not correct this unless you have a HH gyro.

rate gyro - gives a stab of correction (to slow) and returns to neutral position (will drift, you are supposed to do the rest). this is why neutral position MUST be dialed in.

in other words, you must find and adjust the neutral and constant force of the tail...neutral for HOVER + full battery. during spool up it will be slightly deficient and during climb it will exceed. this is where REVO mix comes into play.

as the battery dies, the total RPM will decrease and the tail force will be deficient. This is what makes E heli harder (with rate gyro) than nitro since they dont lose rpm as they use up fuel. you will need to trim rudder as your battery level dies and then reset when you put in a new one.