Well, there's another rule (actually, two rules). It mostly refers to investing (if we consider it a different effort), and it is Warren Buffett's invention. So here are the rules:
Rule 1: Never lose money
Rule 2: Never forget rule 1
If we draw an analogy here, it is not so much about trading the money that you can afford to lose but rather not losing whatever money gets traded. If you follow these two simple rules, you can trade as much money as you want (more money means higher profits)
Probably this applies when your position is in profit
It's six of one and half a dozen of the other
If you are in profit, you are "in the money" (using the futures market parlance). If you are losing money, you are either breaking the rule 2 or, if you don't forget it, the rule 1. But that's not the point. The idea that you should trade only the money you can afford to lose is stillborn, and for a reason. You trade to earn a living, and that means sticking to this rule is meaningless. You could just stay away from trading altogether as well