I'm running the FBL Rotor belt with good results. It's a set and forget belt. I have 144 flights on it already. My E7 is over 11 lbs with Xera 4530 motor and TP 65 5000 packs and the belt can take whatever I throw at it.
It's also kevlar reinforced. I don't know how long it will last, but anyhow I replace all my belts at 200 flights interval, preventative maintenance.
I set it with 4mm left at the end of the boom (protruding from back clamp).