My advice would be 'don't do it'. The cable route is badly designed and it takes the patience of a saint to get even close to maintaining decent shifting after only a few rides. The original cabling on my (non-Di2) medium frame went from standard Jagwire outer to Jagwire Elite (expensive) then back to standard outer. That's just to alleviate the shifting issue. I know Di2 mechs are expensive but IMO the benefits far outweigh the cost.