40-120-x DT volume.
My tanks volume is 140 gal, so in this example it is about 90x my volume.
I run it higher sometimes. At the time it is mostly LPS. If you have Acros, flow should be even higher.
I don't go by gph. I think distribution is more important. On a 90~120g tank I would use four pumps. My favorite set up is two gyres up top where the wires are less noticeable, and two MPs down low. All four on the back.
well if you want 10x turnover for example, then you are looking for 900 gph powerhead. For me, 10x is very low, I prefer about 30x at least.
And remember, a powerhead rated for 900 gph will almost never go 900 gph, they get dirty and clogged up and every day lose a bit more power. So always go slightly stronger.