I hear you Bill.
I'll put together a graph of the pressure drops, for 2', 3' and 4', at 0.5 to 3.0 gpm, for 3/8" and 1/2" ID tubing.
No there is no "set point", it's really down to a design decision. 1.5 gpm is a rough figure I've arrived at, based on the proportion of the pressure drop from the tubing, relative to the total head of the pumps used to achieve those flow rates (reticular implant?).
To qualify my statement, I could compare a few blocks, cores, and pumps, in their various combinations, and present the differences with 3/8 and 1/2". An interesting challenge, but I believe that the wide availability of 1/2" fittings is more important.
Still, might be worth doing...
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