Throw distance = Width of the Screen x the Lens.
First Example: .8 lens 10′ screen needs 8′ of throw. This is to the front of the projector.
This is a rule of thumb. It gets you in the park and has not let me down so far. For an exact figure, look up the projector and the lens. There are web sites to plug in projector type and lens or any combo of information.
100′ throw to a 14′ screen. 100 divide by 14 tells me I need a lens with a 7 in the zoom range. Like a 6.2 to 9.8.