best custom cable length?

I'm looking to get custom cables from DreambigbyRayMOD, what's the best cable length that you have found to work in the case? I want to route the cpu 8 pin at the back of the case through the top left corner, everything else pretty standard with the psu cables coming out from the bottom.

Also, what would be the best materiel to work best (I.E: Paracord, Silicon, Etc)

Thank you!