This is incorrect, as a Mag owner myself I feel I am quite qualified to correct it.
Mags like High Flow and High amount of Light. Once you have provided both of those, Mags tend to stay put. There is no difference to my rocks ending 12" off the surface with 15" below and the Mag planting himself there or there being 12" between the only rock available sitting on the bottom of the tank and the surface. They just want the light.
The question really becomes what's that magic cutoff number of how much room you need to give a mag to the surface of the tank for the location in which they will be attached. This is going to vary from Mag to Mag. As tentacle length varies greatly on just the ones I currently have.