Don't mean to hijack, but this has my interest as well..... I've 'always' used commercial rp screens, never done a DIY rp before.
How does the 'magic' rear projection silver fire mix compare with a typical fresnel-lenticular 2.8-4.0 screen, or even a da-lite 1.3-1.8 rp?
I have a couple old clarity 38's with 4.0 gain screens, and I've verified I can focus up to about 100", however... the closest to clarities single element fresnel-lenticular, was a double element fresnel-lenticular that I got for free from a dead sony crt rp.
So, I've got the makings of a good test platform for rp. Assuming 700 lumens, less than 100" image diagonal, total black out behind the screen, moderate ambient in front -- what's the formula?