They are giving you a dilution of up to 21 gallons. I'd personally go with a full tank when using one as it will be a much longer exposure and should do a better job that way. I'd imagine that the "higher concentration" from less gas in the tank wouldn't be as beneficial due to less exposure time. they list up to 21 gallons as that most likely equals their "prime" concentration/exposure for the additive to do its job properly. it would be one thing if you were spraying the additive onto the internals and letting it rest, but that's not how its working. of course, that's just my opinion.