I have always used a 40 part water to 1 part dishwashing liquid as a spray for spider mites. I use Sunlight dishwashing liquid, the original lemon one. I just put that in a hand sprayer and spray the underside of the leaves. It is best to do it when it isn't too hot outside or if the plant is in full sun. I usually wait 15 to 20 minutes or so and rinse with clear water.
In our area, we have a call in radio show with a plant guru, Ed Lawrence, and whenever I think of the soap solution, I can hear his voice repeating the ratio of water and soap. He must have repeated that on air a million times
