Diameter only comes into play when you need to calculate total pressure or total weight. Then it's psi times area. Remember that 2.31 is based on water at 60 degrees F, if it's hotter or a different liquid you need to factor by specific gravity.
I have to engineer pumping and piping systems fairly often so I have this stuff burned into my frontal lobe.
it gets more fun when you get into calculating suction specific speed and NPSHA. I usually try to pawn that off to one of the junior engineers
