Rule of 500
How high to bid ?
When you are vulnerable, don't bid any higher than a bid which could lead to you going off by 2. Falling short of your contract by 2 tricks will get you 500 penalty points, and you don't want any risk of more than that.
Perhaps you're Preempting, to stop the opponents getting a contact. Or perhaps one day you were thinking "If I bid a bit higher, we might get to game". That would be 600 points if you're vulnerable. Tempting, especially as the penalty for going down is only 100 points. If your opponents are the type that might double you, then the penalty would be 200. Still worth it. If you go off 2, the penalty becomes 200, or 500 if you are doubled. . .. and the score becomes 700 if you make while doubled. Just worth it, if it's a 50/50 chance of making.
But if there is a chance you could go off 3, you'll lose 800 points, and that's just too much.