Seachem Potassium is used to dose potassium (obviously), KH2PO4 is used primarily to dose phosphates. Reaching any suggested potassium level with KH2PO4 would result in a phosphate level well beyond what is necessary (at the very least).
Checking the sticky at the top of the forum will provide recommended dose levels.
this link(v1.0 of Yet Another Nutrient Calculator is live
) provides a calculator to give results.
Calculating from your example...So if each ml of your solution results in .53 ppm of PO4 in 10 gallons of water and you want to dose a 50 gallon to .53 ppm you need 5 ml (5 doses of 10 gallons). If you want to dose a 10 gallon to 1 ppm you use 2 mls, etc.
Under-dose, over-dose, just right tends to be a matter of opinion.