Mutley, I would be more than happy to talk you through your specific setup, but would need more information, such as the size of your sump/refugium and how it is setup. What size overflows and how many? What kind of skimmer are you using? What are you keeping in the tank and do you have additional water flow devices in the tank itself?
The reason you don't see any hard set rules is that tanks are setup differently in terms of size, type of equipment and its configuration and what is kept in the tank. There are too many variables to have anything but guidlelines.
Typically most will recommend a target flow through the tank and sump at about 10x your systems capacity in gallons and another 10x your tanks capacity in the tank itself through powerheads or a closed loop circulation system.
There are a couple of things you are trying to achieve with flow rates:
1. Depending on what you are keeping in your tank you need certain flow rates in order to allow for corals to exchange wastes for nutrients. SPS need very high flow rates, while SPS and softies need lower rates.
2. Gas exchange can only happen where there is an air/water interface. If the water is not circulated enough gas exchange is limited.
3. You need enough turnover in the sump that the skimmer and other filtration like GAC has something to work on. If you have a very effiecient and powerful skimmer with low flow/turnover rates in the sump it is possible that the skimmer might not reach its full potential because it strips the water clean faster than new nutrient rich water is replaced in the sump.
Your equipment has some built in limiting factors as well. For example, a 1" overflow will allow a safe flow rate of 600gph (there is a fudge factor built into that). So before you even decide on what flow through rate you want through the sump you need to make sure your overflows can handle it, or be prepared to mop up some water off your floor.
If you are using a sump/refugium combination how that is setup will play on what flow rate you want as well. These setups can be setup as either flow through or "branched". With a flow through, all the water flow either goes to the sump, then the refugium (or visa versa) before being returned to the tank. or they can be branched in that some of the water from the tank goes to the tank and some to the refugium, before being mixed together again and returned to the tank.
Since refugiums usually work best with a lower flow rate, maybe 5-10x the refugium size you can see the problem with a flow through system. Lets say you have a 100 gallon tank and a 20gallon refugium. That means you would be trying to put 1000gallons per hour through the sump, but you would only want 100-200gph going through the refugium. The closer the size of your tank is to the size of the refugium, the less this becomes and issue. But a branched systems allows you to regulate the system flow through seperately from the refugium flow through.
I've seen so many variations and they all work well, so it really shouldn't matter.
