All manufactured uv's have different flow needs but For aqua ultraviolet 57w classic (is this the one you have?) the numbers posted on brs states
Saltwater Reef Applications: 2133 GPH - 3200 GPH
Protozoa Bacteria Water Flow Rate: 1066 (min), 3200 GPH (max)
For protozoans I would run close to the minimum to make sure they get a good amount of exposure to the uv light.
Always a good idea to consult the manual you received too it should lay out the flow requirements.
As for placement, I've seen the big ones strapped vertically to the sides of tank stands, if you have a big filtration room you can set it up on a fancy manifold, or you can do what I do and lay it horizontal in front of my sump, throw a pump in to the skimmer chamber and have the water exit the uv into the return pump chamber lol