And this is why I make 50G at a time. Just open a bag and dump in, perfect every time.
Typically 1/2C per gallon. Start at 4gallons(2C), test, add a little more so you don't overshoot. When I used to do 5G at a time I did 2 1/4C, test, then add a bit more if need be(1/4C at a time).