KEEP the boom at shoulder height. If you lower it the result will be "messed up sailing" and for example more difficult to plane.
POSITION the harness lines so that the attachments are on both sides of the sail's center of effort. You find the aproximate center of effort by holding up the sail in the wind, like sailing on a reach, and by trying to find a spot on the boom where you can keep the sail upright for a moment with just one hand.
The harness line length should be such that you with some effort can force your palm against the boom when you elbow is the the harness line loop.