IGNORE THIS POST, I THOUGHT YOU WERE DOSING POTASSIUM PHOSPHATE NOT POTASSIUM SULFATE
I mix 1 1/2 teaspoons into 250ml of water. I use old seachem containers. For every 8ml you dose it will increase your PO4 to 1.04 ppm.
People like to target around 1 to 2 ppm of PO4.
I would also suggest getting a test for PO4 to see whats going on in your water from the start.
Just so you know, everyone situation is different, so at first test your water right before water change to see whats going on and don't be alarmed if your PO4 test through the roof, I've had mine as high as 10ppm and higher, with the water change the tank resets itself and you start again adding more or less depending on tests. Before you know it your water chemistry will become second nature to you and you wont have to test anymore. The magic of estimative index.