How do I determine Throw to Width ratio?

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')