There is only one 'rule': Put your harness lines where they are the most comfortable/balanced and then concentrate on trim.
I actually move my lines forward a bit on the upwind side when speed sailing to take the strain off my front arm and tend to keep the back arm virtually straight which also keeps the strain off it as much as possible. On the speed/downwind side I have them where the sail is as balanced as possible so then it is purely a matter of 'feel' for trim with no uneven strain.
In my humble opinion, most of the harness lines available today get too tight on the boom and too hard to adjust for position which makes it hard to make small changes on the water to get the balance just right.
If you can do this, they are perfect: