My total alkalinity is 70 How much baking soda do I add?

A TA of 70 is in the normal range (60-80 with a SWG or 70-90 without), so you don't need to add any baking soda. (The only exception would be if you are going to use trichlor floaters over the winter, which we don't recommend. If you are doing that TA should be higher.)
 
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