Divide the throw distance, from the front of the projector lens to the screen in inches, by the width of the projected image in inches.throwtowidth

Example:

Formula: Throw Distance / Image Width = Throw : Width Ratio

Throw Distance = 10' (120 inches)
Image Width = 8' (96 inches)
Throw:Width Ratio = 1.25 : 1 (10' ÷ 8')