I have been doing a no-no with my reef...using 2 whispers with a powerhead. Dont tell

Anyway I want to build a hang on the back overflow (my tank is not drilled) to a filter-skimmer-refugium-heater and pump return. The problem that has been racking my brain is how do i know how many
GPH the overflow is going to flow to the
refugium so i can match it with a pump. I would like something like 500-600gph but I am unsure about
refugium filter rates. I have a 46g bow front, what flow rate do I need? So how do I know
GPH and what
GPH do I need?