what kind of lights are you running? That’s the key to help placement.
Kind of an FYI i have my duncans on the bed off set under 250 watts 15k bulb.
My undatas are up top 10inches away from the fixture to half way down the sides of the live rock.
You should ask the seller what type of lights it was under. Unless it came from a LFS that just throws them under lights on egg crate racks and moves them out as fast as they can. You should try and replicate the lights it came from. Whether that be moving them up to the top of down away from the lights.
JM2C