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.
) 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.