I'm actually going through the process for the exact same thing right now. I got an Adjustable Ridgeline as a PIF, and started making the measurements. The ridgeline I got is way too big for it, but better too long than too short. The GTUL itself is 9' long, and I think I read somewhere that you take 83% of that to figure out what your ridgeline length should be, so in this case that'd be ~90". Not knowing much about the splicing aspect of it, I'm not sure how much additional you'd need to create the whoopie end of it, for the loop on the other end, etc. Hopefully this is good to start with.
The perversity of the universe tends towards a maximum. -- O'Toole's Corollary of Finagle's Law