Your calculations look good to me, well at least the 7.5% and 51% are good based on you $50k account. To actually put it into a calculator, type 3.24 x 7921 / 50000. But I know you know what you mean.

It puts a different perspective on it. When I turned down my margins I did it such that the total capital required for Z3, Z5 and Z12 would be about 80% of my account. If they all went bad at the same time to the greatest historical extent ( and I didn't stop it on the way) I would lose 80% of my account. At these margin settings, each system made about 200% to 250%. So with some blurring of my eyes I figured I could expect overall 200% return on my account balance. That was my thinking about a month ago.