Best placement is hard to tell. It will depend on what you have where in your tank such as equipment and decorations.
When setting up my 125gal, I put an airstone in my return line before the return pump (because microbubbles show better), turn it on briefly, then check the bubble flow pattern to make sure I didn't have any dead spots in circulation. If so, adjust the direction/location of circulation pumps and try again.
125gal FOWLR, 20L Sump w/LR and Skimmer, 20T Fuge w/DSB, Cheato, Pods and Algae Turf Scrubber