Emma, in about 400 jumps my brakes have shrank almost 3 and a half inches. Talking with John LeBlanc last winter he mentioned that most people set their braks too short on sabres and can actually stand to lengthen them a bit. What I'd do (and am having done this week to my canopy) is having the lower brake lines replaced and made a few inches longer again. I told my rigger to leave about 6 inches more then the current lines so I can play with the right length.
naaa. take it by steps.... if there is deflection when you have your hands all the way up, 3 inches is probably OK. in full flight, the brake lines should arc with no deflection.
Do you do either front riser staight in or carved approaches for landing? if yes, then start with the 3 inches, and play up hih with your front risers. Doing theses manaouvres while holding on the toggles should not deflect the tail. If it deflects the tails, add more inch by inch.
Remember that the landings will be slightly different after each mod, so take it easy...
Is there a more scientific approach I can employ to judge how much longer they should be?
Yes, after lengething them (be careful because that will change where your hands ae during the flare) in full flight, pull one tiggle down about an inch. If the canopy turns, the steering lines are still too short. If you don't use your front risers for landing approaches (hook turns) you only need a little slack in your steering lines. Continue lengthening them (as long as you can still flare) until you can pull one toggle down an inch or so without the canopy turning. Rmember that as your spectra lines shrink, you will have to lengthen the steering lines to compensate because the steering line shrink faster than the rest of the lines.