path dependency is the main problem with margin trading. S^P 500 fell 60% in 2007-2008. So that should give you an idea of how much of a cushion you need to give yourself for the worse case scenario.
if you have have 100% margin and the market falls 50%, the broker will close out all your positions at a large loss to protect its own assets (often well before the 50% target), at which point it will not matter how much the market rebounds after that