To reach a target of 10ppm (on 7 gallons) you add 590mg to the water. One teaspoon of K2SO4 is 6400mg. To mix 250ml solution and use a 5ml dose we need to multiply 50 (doses in 250ml) x 590mg (dose to reach 10ppm) = 29,500mg. How many teaspoons? Divide 6400 into 29,500 = 4.6 teaspoons. Add that into 250ml of water and use 5ml per dose.
I use the API phosphate kit. Seems to work fine.