Acoustic panels - how high?

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.

Elwood +1.

"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."

The problem with that technique (using a mirror) is that speakers do not have uniform dispersion patterns thus making it difficult to visually obtain optimum results (using a mirror). This is especially true if you are listening to all panels at once initially when trying to make judgements. Not to mention all speakers actually have different dispersion patterns, making generalizations about location of panels kind of irrelevant. If you could obtain the dispersion pattern for your particular speakers you can do the geometry calculations and figure out exactly where the panels should go a priori.

oh, and another thing. not all panels are effective to the same degree and some panels do more harm than good. I wont mention any names but one of those that are ineffective is SONEX. it all kind of depends on what happens to the wave as it passes through the panel material twice.