I usually treat each bucket of water refill separately, so for you that might be 1/2 ml per bucket. You can also add the untreated water to the tank and then treat the entire tank, but I'm not a big fan of that method. I like only adding treated water.
Treating the entire tank is a far better choice when using a python or other water changer to do your maintenance. That's what I do.
To give you an idea of how I measure small quantities of prime for my aquariums, I use a small pipette that can measure up to 3mL. In your case, you would only need to use 0.5mL. Having pipettes on hand really make measuring these things a lot easier.
I use about a quarter capful of Prime per bucket during water changes, which equates to roughly a full cap for 7 gallons of water added to my 36 gallon tank. Is there any danger in exceeding the recommended dosage?