I have the 18% grey card and tried matching by eye to the various shades of grey at Home Depot. The paint was for my walls and ceiling. I ended up using Behr "Slate Ridge" Grey. Not close to the 18% grey as it is darker but I wanted to err on the dark side vs the light side for my walls and ceilings.
Just yesterday I took down my temporary large sheets of white paper that I used to temporarily converge my BG808s and also to begin to mount my DIY screen. I projected on the wall painted "Slate Ridge" and the picture didn't look half bad of course the bright scenes were not bright at all but entirely watchable for short durations. In no way would I recommend this as a permanent screen.