I use a 5 gallon bucket when I make my mix, so if I made, let's say, 4 buckets worth of mix, then I would add 20 tablespoons of lime. Container size is frequently described by the gallon, 5, 10, 18, etc. So there are several ways to tell how many gallons you have.

It's not a stupid question, but the answer is really simple.
You mix up a known volume, whether you have a mixing/measuring container that is a half gallon or a hundred gallon.

Also, be aware that nursery container classification - #5, for example - does not typically denote true gallon volume. A #1 nursery container is .71 gallons, a #5 is closer to 4-gallons. Anyhow, adjust your measurements accordingly.

Josh