You running a refugium and sump in series so that somewhat makes it harder to determine as they are two separate vessels with two separate purposes.
If you were overflowing into a sump having only a heater and skimmer easy, I would say just match your skimmer rating.
For a refugium see 5-10x the size of tank used (Calfo/Fenner) and I'm sure someone will say go 20X since a refugium can split into different types. You doing cheato only, thinking about a refugium in it's truest sense (a place a refuge for injured fish or area of no predation), a tank on the side for more delicate species (seahorses) etc. You don't need the flow of a reef tank but you don't want to go stagnate either.
If you had space or mod your plumbing you could split flows. I have planned a separate ref sitting above and to the side of my 75g sump fed either by teeing off one of the drains or off the return pump (most likely) and it would overflow into the main sump.
Didn't get it done yet so currently I have a single sump/ref combined. Using a 75g that has a skimmer in one section, two types of algae each in their own separate sections, areas for LR, heater etc. which works also.
I could reread the thread but guess I'm trying to say what's your goal and go from there. Then again you could drop a pump like
this into your final ref tank and probably not look back. Maybe start another thread with a poll to get an idea for combined sump/ref return pump? Another good reference is Canreef TOTM and read through the setups of some of those great tanks for ideas.
btw I do feel for you. I had started planning my tank before the basement was even dug of the house I'm in now. Every time I finally said no more, this is what I'm going with, I would stumble onto another option. And even now it's setup, I'm still playing.