Acoustic panels - how high?

Rockyboy...I have mine (12"x48" & 24"x48") exactly centered on the wall. 24 inches above and below the panel. I have 8 foot ceilings also.

I built a very crude stand for them that could be raised or lowered 6 or so inches at a time. I started them at 24" from the ceiling and lowered them until I found the perfect spot. Actually, I got them too low and had to raise them up. I went up and down several times with them to get them right. It just so happened to be centered on the wall when all was said and done.

I do have panel speakers (Magnepan MG3.6 & Martin Logan CLSIIA).

Very interesting. Thanks.. I've adjusted mine a couple of times but can't hear any difference. Of course, i can't remember what I had for breakfast this morning let alone how my system sounded before I moved the panels.

stay at your listening position and have a friend move a mirror along the wall until the speaker sound emitting areas are centered in the mirror. This center point can then be used as the center point of the absorber panels.

Like almost all room acoustic treatments let your ears be your guide. Since it is almost completely unpredictable how a particular room will interact with X brand speakers unfortunately the only way to achieve optimum or even good results with panels is to experiment - one panel at a time. I.e., if you try to find the best locations with multiple panels at the same time, with the panels all on in place when you listen, it's like trying to solve three simultaneous equations in four unknowns. Tip: when listening to one panel, keep all the other panels in another room otherwise they'll interfere with your experiment, then introduce one panel into the room at a time and listen again.

Follow Elwood's recommendation.