There is a way to figure out the correct sheeting point.

Before you hoist the sail, draw a line from the corner of the leech and the foot that is exactly half of the angle. For example, if the foot and the leech form a perfect 90 degree right angle, then draw a line from the corner towards the center of the sail at a 45 degree angle from the foot or leech.

Now hoist the sail and attach the sheet. Pull the sheet back along the deck where the track is to be laid until the sheet comes in line with the line you drew on the sail. This will give you a good reference point as to where to start your track.

If you're confused, then I can try to draw up something I suppose.