Hi Cat! Because it can be tricky to tell how mashed potatoes will affect the dough, we'd recommend calculating for the tangzhong starter and leaving the potatoes out of the equation, then adding additional flour as needed to make the dough the proper consistency later. Try to keep track of how much flour you add so on future baking adventures, you'll have a clearer idea of how much flour you need. We hope this helps and happy baking! 

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