The Asphalt Shingle Manufacturers Association recommends 1 to 2.5 gals bleach, 2.5 to 5 gals water, and 1 cup trisodium phosphate. I understand it's readily available at Home Depot in 5 gal buckets in powder form.
Tim Aselton turned me on to the article by giving me a link to the article. It's on this forum somewhere (I just looked for it, and couldn't find it).
Here ya go.
www.softwashing.com/docs/guidelines.pdf