Hi there, Aidan! If you're converting a recipe to include a tangzhong starter, you want to increase the hydration percentage from whatever it currently is to 75%. That's correct that you'll want to 375 grams of milk. Does your recipe have any other liquid or wet ingredients in it, like eggs or oil? If so, using 424 grams of milk might make for a dough that is sticky and a bit tricky to work with. If not, then to get a true hydration of 75%, then yes, you'd want to use 424 grams of milk. We hope this can help and happy experimenting! 

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