well I waited until I got my skimmer before I put my dividers in, That way I knew it was going to fit. I would suggest you wait for your skimmer and then com back to this..As far as placement will I am still just a rookie,,but I made my skimmer section just a bit bigger than my skimmer, the total height of my water will be about 6 inches from the tank rim..you might also want to give the size your your sump you will get more help that way