Acoustic panels - how high?

Is there a rule of thumb on how high an acoustic panel should be hung? I have a number of 2'x4' panels I want to hang behind my speakers and at first and second reflection points. My walls are 8ft tall.
Do I center the panels vertically on the wall or, since the top of my monitor speaker is only 4' high, do I lower the panel and center it on say the center of the speaker cabinet ?