The closer your reference point is, the easier it is to manage. Mine is at stringbed. How high i hit the ball... of course I know exactly at what height is the net when I am at the baseline.
Project the net to the baseline. Get to know the net.
Whenever you are hitting at your rally depth, you use always the same height to stay in rally pace. the same to attack and the same to defend. 3 different heights.. depending on the situation.
Its an association process, takes time, but its actually quite simple if you give time for this idea to settle.
There are no easy quick fixes in tennis, sorry.