The best way to go about mixing paint for a homemade green screen is to have the paint specialists at your local hardware store mix it for you. Unfortunately, the tints and paint colorants are different at different hardware stores, so there is no magic chroma-key formula you can bring to the paint counter. Some savvy hardware store employees may have their own formulas for chroma key paint, but most will not. One way to get around this is to look through the color sample chips all hardware store paint departments have and find the one that looks closest to a classic green screen.
Another way to go about mixing up the correct color is to involve your local graphic designer. Most print designers use the Pantone CMYK 4-Color process guide to define colors. "Videomaker Magazine" recommends Pantone DS 284-3c, which has a CMYK proportion of 60/0/70/20, for a green screen. Look this color up in the process guide, and bring it into your local hardware store. Many stores will have a computer that can scan the color and tell the employees which tints to use to replicate it.