Rule of thumb for my spreads is 10% potential ROE. So, if I'm targeting a 200 dollar gain with a credit spread, my margin will be no greater than 2K...often less as I get more than 10% ROE most of the time (selling 16 delta strikes).  Hard to imagine you're using less than 2K of capital if the trade is going to be worthwhile, considering the systems trades often just shoot for a percent here and a percent there kind of thing.