Anyone have a good reference for distance placement 7.1 speakers?

[krizzz]

My new HT room is pretty long (26 feet) and I'd like to re-use my Infinity towers as rears, but I think they may be too far behind the seating position for good rear sound.

FYI...As the room is laid out, the rear wall is 12 feet behind the seating position and I don't want free standing floor speakers in the middle of the room. I will either be able to position them on the back wall and bump up the rear channel or need to get some ceiling-mount speakers for the rear (but would rather not).

Any idea how I can calculate the proper distances allowed for speaker placement?