I'm in the same boat. I've basically got an entire 3/4" piped system (except the manifolds use 1" fittings and are 1" valves). I'm also going to run a line about 100' and add another manifold for a garden area. Is there an advantage to using 1" for this new line when the water already runs through ~100' of 3/4" pipe before it gets to the new 1" pipe?