Bacillus Bulgaricus starters are very powerful so your yogurt will easily catch. However, if you need some help in figuring out how much starter to use for how much milk, here’s a quick guide to make it easier for you:
Use 1 gram for a liter/quart of milk.
Use about 1.5 grams to make half a gallon / 2 liters of yogurt.
This is about 1/3 of a teaspoon of starter.
Use about 2 grams of starter to make a gallon / 4 liters of yogurt.
This is about half a teaspoon of starter.
Use about 4 grams of starter to make 2 gallons / 8 liters of yogurt.
This is about a full teaspoon of starter.
Note: The minimum recommended quantity of starter you should use is 1 gram or about 1/4 of a teaspoon. That is enough for about 16 — 32oz / 0.5 — 1 liters of yogurt. Your yogurt may not catch or may require much more time to catch if you use less than the minimum recommended quantity.
Here’s a chart that tells you how much to use for your needs
It is possible to split the contents of the packs. After splitting, the gram to quart/liter ratio remains as outlined in this chart, i.e. splitting a 20g pack in 2x 10g will result in 2 batches of starter for 50 quarts each, splitting 20g in 5x 4g will result in 5 batches of 8 quarts, splitting 20g in 20x 1g will result in 20 batches of 1 quart, etc.