On a typical 3/4, I've read the 600 previously and that seems very very low considering most of us run tees or manifolds. That 600 assumption would mean anyone with a manifold would probably get a significantly less GPH actually going into DT if we assume a tee splits it 50% unless you have a fairly powerful pump forcing more water through it. The 1200 makes me feel better lol as this should still get 3-5x turnover even when you factor in a number of lose including tee and manifolds. Given this, I plan to upgrade the initial pump output PVC including first split point at the tee to make sure I get as much GPH before it's force to split and effectively reduce GPH. Still want as much GPH as possible going into the 3/4 split run into DT.
Pump output (1") -> Tee (1", 3/4", 3/4") -> Manifold (3/4") and DT return (3/4")
I don't have a way measure so I will never know. =)