The best placement for it is wherever you like the looks of it. I tend to keep my wavemakers a few inches below the surface but they're gyres so they're not really adjustable and I have them high enough to not cause a huge sand storm. With wavemakers like yours that can be aimed, I usually go either 2"-3" down from the surface and aim it so that it's not disturbing the sand and isn't causing huge disturbance in the surface but still allows for plenty of surface movement to help with gas exchange.
I usually set it, get the surface agitation I'm looking for and then watch the sand.
If it's still stirring sand I'll watch it for a bit and see if it's digging pits and then adjust as needed.
If the sand is stirring you can lower the powerhead and aim it up a little more.
If the powerhead power/speed is adjustable you can always start with a lower setting and slowly turn it up until you find the flow you're looking for.