Hi,
I'm not really sure why you would not be able to do the calculation. If you know the size of the tank, why can't you just divide your tank volume by 20? This tells you how much to multiply the fractions by and tells you how much powder to add. So if your tank is 60G then just multiply the numbers by 3. So this means 3/8 teaspoon of KNO3 (which you can just round to 1/4 teaspoon) and 3/32 KH2PO4, which you can just round to 1/8 teaspoon.
Excess nutrients don't cause algae. Lack of nutrients causes algae.
Cheers,