BenQ MU686 User Manual page 16

For example, if you are using an 120-inch screen, the recommended projection distance is
3425 mm.
If your measured projection distance is 5200 mm, the closest match in the "Distance from
screen (mm)" column is 5137 mm. Looking across this row shows that a 180'' (about 4.6
m) screen is required.
All measurements are approximate and may vary from the actual sizes.
BenQ recommends that if you intend to permanently install the projector, you should physically test the
projection size and distance using the actual projector in situ before you permanently install it, so as to make
allowance for this projector's optical characteristics. This will help you determine the exact mounting position
so that it best suits your installation location.
16 Positioning your projector