I think this one rule (or goal) solves all problems (in it's simplest form):
Margin Level > 1000% - Good!
Margin Level < 1000% - Over Trading, caution! only close trades.
Margin Level < 500% - Seriously! what are you doing? Don't cry when you get margin called, you started it :-P
A smart trader will fund account under 500%, no new trades till over 1000%, Noob Trader will open new trades in hope of regaining margin level by closing them in profit (I kept doing this for 3 years before I learned my lesson - you thought you'll know everything about forex trading in 3 years? ha ha ha! yeah, not going to happen, you'll still be learning new things after 6+ years).
This is the easiest and most effective rule to follow, it doesn't require a rocket science degree, it fits easily on a sticky note (if that's your groove) and you can get some sleep (If you go from over 1000% margin level to bust overnight... you either have a crooked broker, the earth exploded or the sky fell).
For every loss there should be at least an equal and opposite profit.