I don't know if I follow exactly what your plan is, but I would pattern it similar to mine. I would teed off of the main line using copper and a stop and waste valve, then on to the back flow preventer with copper. You could use PVC after the valve. From the BFP, I transitioned from copper to PVC and that runs to my valve manifold. All 4 valves are on 1 manifold. I use PVC pipe for my zone lines as well. I guess you can tell I not a fan of poly pipe. I remove my BFP in the fall and blow the lines will compressed air. The BFV is tested, repaired if and when necessary, and recertified each spring, so I'm good to go for another season.