Its not the strength that matters, its the diamter. I got some 150 lbs magnets for a 36" mx, they flew off. I got large diamter (5-6") 50 lbs and they hold better.
The strength of the magnets is useless if theres very little metal, like a tin roof. Put them on 3/8th" thick steel and theyll never come off.
I got a few what claims to be 3 lbs capacity magnets from a department store and they hold a dashlaser, firebeam and ECCO evolution minibar like nothing.