The flow rate has nothing to do with tank size - that's the maximum flow rate for parasite sterilization. Most of us run the flow slower to get a longer dwell/UV exposure rate. A slower flow of ~500Gph will result in a radiation of over 180,000 µw/cm2 which is actually approaching kill (not stun) levels for parasites.That 1066gph is for a 57 watt uv on a 355 gallon tank. Over 10x turnover is way to much. the parasites would be blasted through so fast the uv doesn't have time to sterilize them.
If you're running a UV on a separate pump just dial the flow back. 57 watts is still *plenty* for a 90 gallon tank (I only run a 57 watt on my 200-gallon system).