I'm not good with this kind of stuff, but I think that it will probobaly have to do with what kinds of livestock/coral you plan to add since in my experience I adjust my flow according to what my inhabitants need, rather than the tank's size.
I can always compensate flow with power heads.
Is it going to be a reef tank? FLOWR? Or something else? Fish atleast usually don't require too much regarding flow, rather surface agitation, but corals are pretty sensitive with it. I kept on having to add, remove, and adjust my powerheads according to my corals' needs when I first started a year ago. Your scape can also affect how your flow is distributed so it may just be on you to fine tweak and find the ideal flow for whatever it is you're wanting to do. Because I had the same issue; could hardly get any information or guidance on "if my flow is good", but theres no real answer to that.
