Cardas speaker placement method provides a good basic idea on setting-up the speakers but does not necessarily yield optimum results in most cases. It all comes down to experimenting yourself in your own room with your own ears/listening preferences to which you feel the best sound you are able to obtain given your preferred choice of speaker placement.
Speaker placement is always governed by room size. 60'' is about 5' apart. Since you asked, if following Cardas method, the recommended distance between my speakers is roughly 5' and this goes the same with the distance of speakers from the back wall. To have the widest soundstage, I have my speakers about 8' apart which are almost touching the side walls, and 3' from the back wall. Room treatments made it possible for close placement to wall boundaries. It is preferable to have the speakers as far apart to have the widest soundstage possible.
The key to achieving optimum speaker placement is in trial and error, and this may be an arduous task for some especially if your speakers are huge and heavy.