I am considering a Sony VLPHS10 DLP Projector. This unit should be avialable soon in the US. Does anyone know how to calculate the focal length? IOW, if I know where I want to mount the projector and how far away the screen will be, how do I calculate the screen size? Conversely, if I know the screen size and where I want to mount it, how do I calculate how far away the projector must be mounted? Or, do these DLP projectors have variable focal lenghts? If so, is there an "optimal" focal length? Thank you for your help. Craig J.