How do you substitute stevia for sugar when baking?

when you buy "baking stevia" it should come with a conversion chart, but i think its probably pretty universal. this is how mine goes: 1 cup sugar = 1/4 cup stevia 1/2 cup sugar = 1/8 cup stevia 1/3 cup sugar = 1 tablespoon and 1.5 teaspoons stevia 1/4 cup sugar = 1 tablespoon stevia