If I understand you correctly, you're just asking about how much water your return pump should be pushing. I've heard 4-5x your total water volume used as a recommendation for a return pump, and that sounds like a pretty good rule of thumb to me. It'll give you a good turnover rate between your sump and DT, allowing your skimmer and whatever else is down there to do its job. The relatively low flow will help keep things quieter. Also, if you plan on making part of your sump into a refugium, you generally don't want a real high flow through that area.