u/mantis-tobaggan-md - typically a good starting point is that you want to sit at the apex (point) of an equilateral triangle where the length of the sides equal the distance between your speakers. So, you'd be sitting the length of the long side of the right triangle with a base length of 1/2 the speaker-speaker distance.

If your speakers were 10 ft apart, then you should sit a little over 8 1/2 ft back.

The listening position P is the distance determined via the following formula based on the speaker separation s:
P = sqrt(s^2 - (s/2)^2) [square root of the separation distance squared minus half the separation distance squared]