have depower on full power and pull the bar all the way in...then the kite should pull the most. If it starts to fly backwards, the back lines are too short. If the kite does not pull at all, back lines are too long.
First adjust your depower length, then trim like that with moving the lines under the swimmer to different knots or on the pigtails on the kite.
If you do this, you have to keep one bar for every kite. Or change every time.
If you do not want that, the way to tune the bar is to keep all lines same lenght.
Toby: If you tune the way you say, do you do this with light wind. Because in light wind you will notice backstall first?