Jake's answer is spot on. Unfortunately some jibs don't have multiple adjustment points in the clew. Some others like the jib I am working with on a N20 as crew don't have a sweet spot. If I run in the normal position I have too much twist at the head. Moving 1 hole up and the sail is too flat. On this particular sail, I will use the lower hole to about 10 kts of breeze (just double trapping) as it gives the sail more belly. I go one hole up (contrary to what I would do with a new sail) as the breeze builds. This flattens the sail and prevents leech flutter upwind in bigger breeze. Again, not what I would do with a new or nearly new sail but its what works on this particular sail. Normally you would go one hole down to open the leech up and de-power the top of the sail, but if you don't have full battens or a stiff leech doing the opposite has proven faster.

Good luck.