The bottom line here is that you are going to have to set the lights by a trial by error method. If I read your dimensions right, you have an aquarium that is 36" long, with one LED unit. Depending on what reflectors are in your light fixture will play a huge role in determining the spread....You will want to center the unit at an estimated height of whatever you want to start with.... 8" will be a good starting point, and then you will have to see of you have dead zones on the corners of the aquarium. You will want to raise it or lower it till you are happy with the spread. Basically that is about it. You may want dead zones on the end for certain corals, not sure if you do. I have 4 Led units over my display and they are 6.5" off the water. I basically did the trial by error method until I got the best spread allowing me to work under the lights as well. The more units you have, most likely the closer to the water you can get, and increase PAR.
