This should be a personal thing for the student , take them up to a safe altitude and from the normal Vref at a normal approach ROD with the gear down and the flaps set for the normal approach setting for the aircraft get the student to "go-around" and observe the loss of altitude before a positive rate of climb is established.
Add the loss of altitude to a safety factor of lets say 200ft and you have a basic number to start with. more demanding airfields may require an additional safety factor.
The altitude that you select for this demo should reflect the aircraft performance at MLW WAT limit.
This will assure that the commital height is within the ability of the student and as more practice is undertaken the exercise can be repeated to lower the limit as the students performance improves.