According to the Directions on the Petco website you should be doing 6 capfuls: "
Shake well before using. Dosage: Approximately one capful treats 1/2 gallon. Use with every water change."
Now, when you're almost done with that conditioner, consider buying Prime or NovAqua. These are great conditioners and they use much less per galon so you'll get more for your money.
The NutraFin is goiing to last you, what? A month?
NovAqua is only 1/2ml per galon and the Prime is 1or2 drops per galon. They will be twice as expensive as your current conditioner but will last you for a LONG time.