a good way to learn is idd by lowering the boom untill you're comfortable. But seeing as you already sail with a harness i don't think you'll find it too hard so I suggest you give it a try with your standard settings.
As for the position of the lines: it's a feel thing. When your planning and you feel that you have to pull really hard on your back hand, move the lines a bit more towards your back hand, same for the opposite effect. There is no standard setting for your lines really. But what does help at first is setting them further appart. To begin with you could simply but the two bits about 1,5 fists appart with the first end about 1,5 fists away from your masthand.
hope this answers part of your question
Haiko, AKA crazychemical