the recipe on the link for the butter says to use 3 ounces trimmings, less if using buds, so i'd say like an 1 1/2 ounces should do, depending on how strong you want them to be.
3.5 grams of bud is about equal to 1 ounce of leaves. you can extract 3.5 grams of bud into 1 quart of milk or 1/2 cup butter to end up with around 15-30 doses depending on desired potency. you will have to experiment for personal preference.
i updated the cooking page with the new dosage information. if you stick to consistent starting amounts your end product will be pretty consistent thus allowing you to know your favorite dose without having to test run each batch. http://www.growkind.com/cooking.html