As a starting point, the jib lead on any sloop should be set up so if it continued up at the same angle, it would bisect the jib luff 50:50. This gives even tension on the foot and luff, and is a good position for most upwind work.
When reaching, you typically move it forward as the sheet is eased, so the top of the sail does not twist off completely.
When the breeze comes up, move it aft to do just the opposite; allow the sail to depower by twisting off.
FUN FACT:
Often on jibs you will see a contracting colored line streaking upward from the clew; thats a trimline placed there by the sailmaker to mark that exact line. This sail is slightly de-powered.