Speaker distance and tow in?????/

My room is big / 18x30. But due to arrangement I can only bring out my speakers 2 ft from back wall..... side walls not an issue. My 2 main L-R are 8 1/2" apart ( center to center of speakers ) . How far back should my seating be? I "think" there is some "basic" math to work from for far field seating. In my "near fields" it is all equal.... as a rule of thumb. Tow in is another adjustment. I am just looking for some "basic" good info and hope I don't start something... You guys have spent a lot of time on these issues..... and I am glad to have your suggestions!