HP Mp3130 - Digital Projector XGA DLP User Manual page 18

Setting Up
If you do not want to use the distance table, you can use the following equations to
help determine where to install the projector relative to the screen.
Calculating projection dimensions
projector lens below bottom of screen = C × width of screen (for 4:3 screen)
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distance from lens to screen = A × width of screen
minimum and maximum distance from screen = B
width of screen = 0.8 × diagonal of screen (for 4:3 screen)
A = 1.67 to 2.00
B = 1.2 to 12 m (4 to 40 ft)
C = 0.21 to 0.45
User's Guide