the inverse square law problem isn't different for short-throw projectors (and it seems the person you are responding to was in fact installing short-throw projectors anyway).
short-throw projectors are just normal projectors optimized for very large beam angles, so at the end of the day the brightness out is exactly proportionnal to lumen amount between regular and short-throw projectors, modulo negligible airborne particles loss.
short-throw projectors are just normal projectors optimized for very large beam angles, so at the end of the day the brightness out is exactly proportionnal to lumen amount between regular and short-throw projectors, modulo negligible airborne particles loss.