Here is a link for a first reflection point calculator:
http://www.acoustic.ua/forms/calculator4.en.html
Although they use cm’s for measurement but you can put inches in the boxes and get the same results. Using this greatly improved the sound quality in my room. I had my reflection points all wrong using a mirror and far too much on the front and back walls. Correcting with the calculator and tuning the room took my system up several notches to the WOW point...saved big $!
Enjoy