Feed the UV should be from he cleanest water available, which in your case is the skimmer our feed side, before your refugium. Do not put after you refugium or in your return section as any critters on the way to your main display will be killed. I also suggest putting the output of the UV to the skimmer section. This allows at least a chance that anything the UV kills may be picked up by the skimmer. While you might think it will be filtering the same water over and over, the inflow/outflow will ensure enough new water is always available. I would also not suggest running the UV all the time unless you have a problem. UV kills everything, including the good stuff in the water column. You want to give the refugium and your live rock a chance to have that good stuff and not strip the water too much.