I agree with Reflex's comments. I've never even heard of the 1/3 rule, windsurfing should follow "if it feels good do it" As Barn mentioned leave your velcro loose a bit so you can adjust on the fly. If your front arm is getting sore move the frontline or both slightly in that direction. Likewise for back arm.
The width you choose to have your lines apart is likely to depend on the style of sailing you do or what feels right. Wave/Freestyle, tend to have them narrow set to allow more room to move hands along boom freely. Speed/Slalom, I have mine around armpit width apart, long enough to have my arms comfortably outstretched whilst maximising leverage over the rig and unweighting the board. This also allows me to slide along the harness line more so than if I had my lines closer together to asist with going upwind. Also it helps with stability/control of the rig especially when your doing around 90kph
In general I think too many have their lines too short regardless of their stature/size. I use waist and seat harness's and adjust my lines accordingly when I use either by around 2" in length. Every time I sail and depending what sail I have up and what board/fin, how the sail is set usually requires an adjustment of my harness lines to get things feeling right. ie more time Rigging/tweaking less time Friggin"
