K2SO4 is potassium in the form of potassium sulfate, you may or may not need it because the N and P provides some as well. personally I think it's a good idea, K is virtually impossible to dose too much. You should add roughly as much as KNO3, or a bit less.
Personally I like scales because you can do a better job of dosing specific ppms of everything, also easier to make precise adjustments. But it really doesnt matter, generally speaking there's no need to get all that specific to begin with.
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