I figure mine using in-tank turnover rate and tank size. You can add in the return pump outflow if you account for head pressure.
So let's use my tank as an example.
120g DT. Mixed reef, mostlly SPS
2 x Maxspect Gyre 250's that max at 70%, ramp from 10% to 70% and back down (any more than that and I get a waterfall over the side) = ~4200gph
2 x koralia 1500gph on alternating timer = 1500gph
2 x koralia 850gph - both on max = 1700gph
That makes my total average in-tank flow ~ 7400gph
7400/120 = ~62x average, with peaks higher and lows lower.
I consider that to be relatively high flow
Medium flow would be in the 30-40x range, and low flow 10-20x.
That being said, there are areas in the tank that are very high flow, others that are more protected by the rocks where the LPS are happy.
What you need to avoid are any dead spots where detritus can accumulate. The pair of 850's are positioned to blow behind the rocks to keep detritus from piling up.