for most situations it probably doesn't matter all that much how exact you have the zero thrust. It's the drills and procedures that matter most.
The usual simulated engine failure procedure in a piston twin is throttle closure. Mixture cuts are potentially dangerous especially if the engine fails to start immediately you select mixture back to rich. Once the problem is identified and the pilot simulates the drills the instructor then sets zero thrust which simulates a feathered prop.
However - the danger is a simulated engine failure after take off - in other words low altitude. When the instructor closes the throttle against the aft stop it not only simulates a windmilling condition but it is actually windmilling and that means lots of drag.
If the student faffs around going through all the "mixture up - pitch up - power up etc drills he must remember the windmilling prop ensures the aircraft will not only lose speed rapidly but also go downhill. Which is precisely why twin engine aircraft have a prop feathering system to remove windmilling drag.
While the exact manifold pressure and associated RPM to simulate zero thrust will vary with conditions, it is vital that the instructor applies zero thrust within a few seconds of pulling back the throttle to its stops to simulate the engine failure.
Because speed/altitude/directional control and climb performance is so critical during simulated engine failure immediately after take off it becomes a serious flight safety issue if the instructor delays setting zero thrust while waiting for the student to go through all the drills up to when he would normally press the feathering button or pull back the pitch lever into feather.
As long as the instructor gets some power into the simulated dead engine literally within seconds of pulling back the throttle in the first place it doesn't matter too much if the zero thrust setting is not exactly correct - as long as the setting is not LESS than zero thrust. Far better to err on the cautious side.