John, as Jay pointed out, even if they're mounted on the back wall, they're still playing side surround material in a 5.1 setup. A rough rule-of-thumb would be 2' above and 2' behind your ears(to lend a little "phantom" back effect in the absence of actual back speakers). As side surrounds they should be separated as widely as possible, maybe 6" from the side walls, rather than the 24" you mention.

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Enjoy the music, not the equipment.