There is no ratio between TX and RX.... The 225-0-225 scale of the programmer is a scale in of itself. The TX transmits 900-1500-2100usec (end-point/center/end-point).The servos react to the signal transmitted and subsequently decoded by the RX. The servos once programed to a specific center and end-point require a signal with a like value pulse width to duplicate the programmed parameters.

If you program the servo to move to a given end-point, this new programmed end-point is at the extreme end of the pulse width range (900 or 2100usec) and reacts to same. You can not realize this pulse width at 100% ATV, you need to max out your TX to realize this signal value. 100% ATV likely transmits a signal value around 200-300usec below the required value to drive the servo to end-point.

Transmitters don't usually deliver a neutral/center pulse width signal value of the desired 1500usec, design parameters, acceptable mechanical, component and bench tolerances typically provide a wide cross section of values, 1518usec is not unusual or anywhere in between for that matter. Of course 1500usec is best as this allows for linear values either side of neutral.