Flow through bulkheads depends on a number of factors:
1. differential pressure across the fitting.
2. The Cv of the fitting (how much flow a 1 psid will cause).
Cv is a common measurement in fluid dynamics. Pronounced "see sub vee" or dimensional loss coefficient or dimensional flow coefficient.
Other things to consider: is this bulkhead a vertical fitting (through the bottom of the tank), is this a horizontal fitting that is not going to have full pipe flow? lots of factors.
My advice: go with the largest fitting possible (schedule 80 pvc). It will provide less head loss and you can always size it down to whatever pipe size after the fitting. If it is for horizontal overflow, remember a vac breaker.
Just my thoughts. Feel free to ask more questions. Maybe I can locate some on-line info, but it is probably way beyond what you need.
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