Mags look for the highest lighted location on the rock structure they're on, if you have a light that offers good coverage and can offer decent PAR to the depth they are at, they do not need to be on the tallest structure in your aquarium and will not wander off -- generally speaking of course. I would make sure the spot intended for it also has good flow, a healthy mag can take quite the beating. Mags are easy to keep where you want them if you specifically create a spot for them that meets both flow and lighting requirements. Unlike BTAs they shouldn't wander too much if you offer them those two things. Getting a healthy one is the hardest challenge, at least stateside, most of them require immediate treatment as they get infected during transit from the elevated ammonia levels.
Edit: Just to clarify, a mag will wander if it can touch ANYTHING other than the structure it's on.