I'd forget about trying calculate flow rates based on the size of your tank and sump. Buy a UV rated for your size tank and adjust the flow to about 80GPH or less. The slower the better as long as you do not have a big problem with heat.
Your post above is a little old but I wonder where you got the hooks for your 150 watts UV unit which is 71" long and 5" diameter if I am not wrong.I have the Emperor 150 watt Smart HO UV as well, and I installed it because I have 6 tangs in the tank. I flow about 500 gallons and hour through the UV. Total volume is about 450 net gallons. A good rule of thumb is one times the total net tank volume flow an hour to get the exposure strength to kill marine ich. Tank is 465 gallon SPS tank and a 155 gallon sump. The UV has a separate pump that pulls water upstream and returns it downstream, so all the water coming out of the UV goes back into the DT. The way I have it installed makes it easy to remove the quartz sleeve for decalcifying and bulb replacement.
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There is absolutely no reason to just set your flow rate to 80 GPH or less and just leave it there. You set the flow rate to the proper exposure time to kill marine protozoans, like marine ich, and anything slower than that is wasteful of electricity. You want to process as much volume through the UV at the proper exposure level as you can. If the particular UV you use has the proper exposure level at a flow rate at 80 gph, then fine, but if you are getting the proper exposure level at 250 gph or whatever the rate is for the wattage UV you use, then go with that.
Based on the documentation do you choose the suggested or maximum for dosage (ie. 180,000)?
I have the 40W unit and I feel Emperor Aquatics makes the best UV sterilizers for the hobby. I have an AquaUV on my other tank and it pales in comparison.