From what you are describing, any flat neutral gray paint would work. If you truly do want just an .8 gain screen, then I would just pick a neutral grey flat and use that. You might want to trim it with a frame of black felt as a border, but other than that it should work just fine. Fore me, I use the BenQ w1070 with a 120" screen, I wanted a higher gain gray screen for use in ambient light, so I went with the silverfire 2.5 3.0oz paint mix that you can find in this DIY forum. Performance in a dark room was better ( more WOW ) with the bare white sintra that I painted it on, but the gray silverfire definitely outperforms in ambient light , which is the reason I chose it.
If .8 is all you really want or need, then don't waste your time with fancy mixes, Just spend a little money on a grey paint and your done.