WHEN PRE-EMPTING
Count your expected winners (assume average suit breaks) and then over bid (pre-empt) by 2 tricks with unfavorable vulnerability, 3 tricks with equal vulnerability, and 4 tricks with favorable (white vs red) vulnerability.

The rule of 2, 3 or 4 presumes that on average you will find one trick in partner's hand.

Sometimes, of course, you won't. But then, that's bridge, isn't it.