Say $10K position, 2:1 Margin, 10% Stop loss on all positions, so look at 16K total position value keeping, leaving you 2K so SL and Margin same approx level with some slack.
Depends on what you mean by "margin". Reg-T margin is simple, and what I use. However, if you are using SPAN or Portfolio Margin, you must be aware of how they are dynamic. Also, may be a function of how you plan to trade (wild man, or measured).