Hardware stores don't usually carry those threaded barbs, in 1/8 increments, at least not in my area.
As for the McMaster part, it's# 5372K218 (nylon). (3/4 FNPT by 5/8 barb). They don't seem to have the brass equivalent.
I can't comment on 3/4 vs 5/8, but I can on 3/4 vs 1/2, to a certain extent: it depends on the flow you achieve. I was hoping to get to 4 gpm, but I was over-optimistic. At that flow rate, 3/4 ID tubing would have been a necessity, otherwise the tubing would be a significant restriction within the whole loop.
At 3 gpm, 1/2 tubing will have a pressure drop equivalent to 0.2 inches of head. 3/4 will drop 0.07 inch. This is all for 1 inch of length of tube (!).
At 2 gpm, 1/2 tubing will have a pressure drop equivalent to 0.1 inches of head. 3/4 will drop 0.01 inch.
At 1 gpm, 1/2 tubing will have a pressure drop equivalent to 0.03 inches of head. 3/4 will drop 0.004 inch.
These were calculated using the Hazen-Williams formulae, and as I found out recently, it may not be accurate at all.
:shrug:
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