No more than 3ml/gal. But that's after its well established. Before that I dilute it. I'll add RO water until I get to my desired PPM. The first dose would be at 30-40 ppm. Then slowly work up. It was very odd I'm still not sure if I am right in my assumption of phosphorus deficiency. I also added 20 ppm of cal/mag. (Before diluted) because it looked like several deficiencies. For whatever reason I can't look at the leaf deficiency picture chart that is floating around the net and decide what the problem was. They all look fairly similar to me. So I figured added a little cal/mag couldn't hurt.