I guess 1 gallon for 300 gallons is a lot, but it works. After a week or so, you can't smell any trace of bleach. I've taken many showers with water from that tank and you don't smell it or get that slippery feeling on your skin like you do when bleach gets on your skin.
I always add a cup of bleach to my 300 gallon tank, but I've heard that pool chlorine tablets are good as well.