There are many online calculators that can help with this. A general rule of thumb is that 1 tablespoon of bleach in 1 gallon of water will yield a solution that is approximately 200 ppm in available chlorine. It is important to note that this is true if the starting bleach is normal bleach (5.25% sodium hypochlorite which is equivalent to 5% available chlorine). Today, bleach is often sold in concentrated form (8.25% sodium hypochlorite which is equivalent to 7.85% available chlorine). It is best to check the label. Another consideration is the age of the bleach. Bleach will lose potency over time, especially if the bottle has been opened several times.