Rear Projection Screen Questions

[ImPulSive]

First off, not sure what group this question belongs in, so if I have posted it in the wrong group, mods please move it to its proper home.

I have a couple of questions about rear projection screens. With Halloween only a month away, I am toying with the idea of using my PJ to set up a display for the kids. I do not have any suitable locations in the house via windows so I was thinking about using the garage area, putting up a bed sheet or similar across the open double wide garage door and doing a rear projection from inside my garage. Thinking about going to hallowindow.com and getting some videos from them. My questions are about the rear projection screen material and brightness.

Material:
First thought was to go to a fabric store and get something that is white and thin, but could also try some cheap white king size sheets too. Thoughts? Best material to use that is also cheap, for a once a year usage model?

Brightness:
There are not too many street lights on my part of the street, so barring a full moon and/or lots of lights on the neighbors' houses, I am hoping it will be sufficiently dark that I will not need a light cannon for this rear PJ. Have an Epson 8700UB that Epson claims is 1600 lumens. Using the calc at PJ Central, I think the biggest screen I could project from inside my garage using maximum zoom would be 95 inches high by 168 inches wide, 193 diagonal. Using a screen gain of 1.0 that is 14 fL. Using the same calculator for my set up in my home theater I get 28 fL with a 1.4 gain screen. Not sure how bright this will be via rear projection. I could also make the screen smaller to increase brightness, but I'd love to use the maximum screen possible.

Any ideas what a rear projection screen gain factor would be so I could enter that into PJ Central calc and check it out? I guess I could always do a test run too, but too busy right now and weather is rain and more rain and wind.

Suggestions?

Thanks.