Can we forget about the BFP for a minute? This is way sidetracked with backflow, and we haven't even gotten there yet. It doesn't matter that some people live on hills above you, the OP doesn't. But no matter, this is all beside the point.
Where does the pipe terminate away from the pool Mrelecric? Is it terminated at the pool equipment pad, or just stubbed out? I must admit, knowing the answer probably won't help me much as this is different than anything I've heard of for Autofill. Usually, it's just a 3/4" line or less piped to a float as mentioned. I'm having a difficult time trying to imagine how this set up is designed to function, but I am very curious. Did the PB give you any clue as to how this setup was supposed to work?
EDIT: Thanks Bama, you beat me to it while I was typing. Can you tell us anything more about this?
This is my learn something new every day item.