outer routing brakes

I would like to install the outer brake system on my FF3
Im a loong time BASEjumper and is used to jump slider off/free brakelines,while flying my paraglider i feel restricted by the guiderings as the glider comes with..

I therefore plan to do the outside routing as i see many pilots fly with..

I however dont seem to find info on the setups,im thinking about the lenght of the elastic fabric how much force does it need to provide at max streach and how much force does the input require..
I need to figure out so im not making a set up flying on a system with too long ride so i cant reach the toogles in midair it i let go,but also that i dont fly using brakes all the time..

How did you guys find the correct lenght of this.

Thanks in advance