I like to sit a little further back than the distance between the Mains.
My understanding is that the mixing engineers sit in an equilateral triangle as that arrangement give the best way to hear the separation between L and R. Sitting a little further back in an isosceles triangle allows for a blending of that hard-stereo separation.
Regardless...
More to the point of your question, the Center should just be, well, centered... between your L and R.
Depending on your room and seating distance, I would say an absolute minimum distance would be with the mains 6' apart on center (middle of front baffle to middle of front baffle, L and R).
Most will say that 6' is far too narrow, and I would agree, except that you still have to take into account what your space allows.
8 or 9' apart is more likely the optimum distance for most, especially, if like me, you don't jam speakers right up against side walls. I like at least 2' between Speaker and side walls, and more if possible.
IIRC, I've seen a few argue for more distance still between L and R mains. I think TLS recommends 12'(?) if possible.
Again, so much is dependent on your space. We all have to make the best use of what we have.
