I place my power heads the nearest I can get it to the surface without it sucking air and also facing up slightly without creating so much turbulence that the water makes noise.
You then need to balance out the power with how far you are trying to throw the water. if you have a really powerful wave maker you cant expect to point it to the shortest opposing glass face without it moving the sand around, so look at 45* angles down the tank or the full length of the tank with your most powerful ones and then blunter angles for your less powerful ones.
You also need to play around with the placement to get no dead spots and also not blast any corals directly, this does take some playing around.
Although not its intended purpose, I find KZ's coral snow really useful to see how the water is moving in the tank