The easiest way and fairly accurate is to take your genoa out onto a large flat surface and fold the head to the tack and mark this possition on the luff. Then take a string from that mark (1/2 luff) and bring the other end to the clew. Take a marker pen and trace where the string is at the clew for about 2' from the clew.
When you put the sail up on the boat line the sheet up with this mark and that is where the designed lead possition is. This will change in different winds. If the boat is over powered or the main is starting to back wind excessively then the lead should move aft.