Check the rigging guide for YOUR sail model and year. That's the easiest, most obvious, AND best answer! Not all sails are supposed to be rigged the same way. Even the same model often changes from year to year. If you don't have a hard copy, look online.
You won't believe how many 'wannabe know-it-alls' you'll find on the beach. Alot of times, they haven't even read the rigging guide for their sail, let alone the rigging guide for your sail. The most believable source is the manufacturer. There are folks you will meet who will claim they know better than the manufacturer. It makes me laugh.
My favorite sails have visual indicators built into the sails. Ezzys, for example, have indicators for correct outhaul AND downhaul.
Lastly, you may want to consider going beyond recommendations in some cases. For example, if you're already at the minimum recommended downhaul and outhaul for your sail, and you're still underpowered, you may want to consider reducing the downhaul and outhaul a little more before rigging a bigger sail. You've got nothing to lose.