Also when buying bleach look at the strength and also the age, date code will be on the packaging for all but the cheapest stuff. The cheap stuff is often 1% or 3%, brand name is generally 6% though 8.25% "concentrated" bleach has been getting very popular these day. You also want plain bleach, not scented, not outdoor which has detergent, not "easy pour" which has thickeners. Bleach looses strength with age, so you want to make sure the manufacture date is sometime in the previous few months, if possible, and not something that has been sitting on the shelf for years.
As to the exact amount to use, sanitizing with bleach is a function of concentration plus exposure time, most things are killed by very short contact times with moderate strength bleach, 1 ppm of chlorine will kill most stuff within a minute or two of contact. There are however exceptions, things that create bio-films for one, as the bio-film creates a protective barrier, though these generally succumb to a sustained 2-3 ppm solution of chlorine within a few hours. The real problem are highly chlorine resistant things like Giardia and Cryptosporidium these hard shelled parasites are highly resistant to Chlorine, as it takes Chlorine in the 50 ppm range several hours (around 8 ) to kill Cryptosporidium (20 ppm chlorine will also kill it, but takes about 2.5 days of contact). Of course Giardia and Cryptosporidium should only be an issue if you connect to contaminated water sources.
Now for the simple answer 8 ounces of 8.25% bleach in 100 gallons of water results in about a 50 ppm Chlorine solution.
Personally if dealing with a known Giardia or Cryptosporidium contamination I might use Chlorine Dioxide not bleach to decontaminate, as it is far more effective against these organisms, though it is also far more expensive.
