In the proposed method of twisting until 10% and then twisting more until the torque setting of the screwdriver is reached i think you'll end up with a shorter leader then you expected. All the "extra" stored energy will be used to furl the legs, so they'll get shorter.
As for the 10%, that's about the best you can get with nylon monofilament with some safety margin. For polyester thread (guetermann tera 180) it's possible to get about 30% reduction. This leader is stiffer and gives some trouble putting shorb loops in it, so i'm thinking of settling for 20-25% with this material. Don't know yet what is possible with fluorocarbon.

Karel