Generally speaking, your skill (or other game value) is a number you want to have high, and then you want to roll low on the dice to get below the skill value.Is it roll under as in a percentage chance to succeed or roll low as in THAC0/descending AC?
A possible twist is "Pendragon"-style rolls (I've also seen them called Blackjack rolls) where you want to roll below your skill, but otherwise as high as possible. This is usually used for opposed rolls and sometimes for other rolls where quality of success is important. It works best with linear randomizers (single die), with bell curve randomizers it usually feels better to use margin of success.