Using Chuck Gadd's nutrient calculator, with your mix, you would be adding .19 ppm K2SO4 for every ml of solution. Suggested range is near 20 ppm. I would personally add a dry teaspoon of K2SO4 directly to the tank. That would give you 17.84 ppm K2SO4 assuming 0 ppm prior.
Do a search here for that calculator; it is really helpful.