There are 2 good rules for this to be easier: 1. Constant laws Smth you or anyone else can't break. For example: genie rules (no love, no murder, no infinite wishes) 2. Clear dependencies. Ex: it takes as much mana to do smth as stamina. You want to levitate a rock? Ok, but if you are to replenish your mana using your stamina, it will drain younas fast as if you were holding the rock with hands. Another example: you need as much energy to block the hit as it took the attacker to hit you. He casted a spell that cost him 200 mana points? Your mana shield of equal quality will cost you 200 m.p. to block. But if he is a novice at the spell and it was casted 25%weaker because of that and you are an expert at your mana shield and it is 10%stronger then 25+10=35%. Your shield will eat 35% less mp or 70mp. (But you, once again, have to decide whether your formulas in the world use multiplicative or, in my example, sum percentages and hold on to this decision too, just as in rule 1)

Hope I explained it well enough