Maybe I am overthinking this. I guess I just don't know how to use the existing system as a guide- it was a bank owned house so I have no contact with the previous owner or builder. I know how many heads are in each zone of the existing system, and that each zone has a 1" valve and it looks like 1" lateral lines, and that the heads are Hunter PGPs, but don't know what the nozzles and flow rates are (they are all 20' or less apart head-to-head and have red nozzles- seems overkill?). I'd like to get some numbers in my head so I can intelligently plan the zones in the backyard to have sufficient coverage without unnecessary zones or heads.
I have tapped into the 1.5" supply line at the existing valve manifold, but I have ~300' of 1.5" pipe between that existing manifold and the new manifolds in the back yard. Seems like that is just semantics whether it is an extension of the existing system or a completely new one