I currently use a 50/50 of baking powder and baking soda, in line with my flour measurement. So, 2 cups flour would use 1 tsp each.
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The amount of baking soda is relative to the amount of acid, which here is buttermilk. For every 8 ounces of acidic ingredient, use 1/2 teaspoon of baking soda along with your baking powder.