Michael,
Slate is surface, like Cedar, that has different factors than a regular Shingle Roof. 3-4% mix and let it dwell, but you will need to rinse the crap out of the roof. SH can leave a funky residue on slate if not rinsed, plus you have some moss build up between the slates. Also, looks like no gutters, so rinse thoroughly and divert and protect the plants and the rest of the house. The copper valley flashings will loose their green patina appearance and most likely turn back to a more copper/orange color. The patina green will come back, but it will take months. Some of the slate looks like there is some orange discoloration, which is the iron ore in the slate tile itself. This orange may lighten, but it may not. Just set your customers expectations. And DO NOT try to walk slate roofs. Spray and rinse from ladders and the ground. Hope this helps and good luck!!