I would just 2 gallons worth, according to the instructions on the back of the bottle. If you are having trouble measuring, pick up a baby syringe from the drug store. They have millilitre gradiations on them so they are awesome for measuring fertilizer, water conditioner and even medications for anything under 20 gallons.
Those are some weird instructions...
Did you by chance buy betta specific water or water conditioner? Those are scams, really...any water conditioner is safe for bettas, and will give you better instructions (cheaper too!).
For future reference, you can just usually use plain tap water. Those 'betta' water bottles are a rip-off. Just make sure you get a good conditioner. I use NovAqua, and a lot of forumers here recommend one called Prime.