One way to get your harness lines in roughly the right area leaving you only have to make a very slight adjustment once out on the water:
First we will focus only on the back part of the harness line (the line closest to the back of your sail). To get you started you want the position of this line to be as close as possible to 1/3rd of the distance across your sail from the mast..
Once you have found this spot for the back line setting the front part of line is easy. Simply measure approximately 5 inches from the back line towards the mast and attach the front part of the harness line (This should be roughly the same size as you fist).
The only times this will not work and you may have to use a different technique is with sails at 7m or bigger. Also shorter people (under 5 foot 6 inches). For either of these two situations as a rough guide you would want to move the back and front harness line connection loops about 2 inches further forward.
Rig up your sail as usual make sure you have a nice shaded spot out of the wind. With the sail flat on the ground pick it up by the boom using only 2 or 3 fingers. The aim of this exercise is to be able to pick up the sail so that it remains horizontal and balanced. Once you find this sweet spot you want to position your straps either side of this balance point, again focus on one fist distance between the two connection loops and remember this technique will still need fine tuning on the water.