Hi Mabel, that is a tricky one! The way I calculate it the hydration of your dough as it currently stands is about 59%, so to increase the hydration to 75% you would need to add about 69g of milk to the dough portion of the recipe. To make the tangzhong starter, I would go with PJ's standard amounts (23g flour and 113g milk), which are both subtracted from the increased hydration dough amounts. That would leave you with 397g remaining flour and 76g remaining milk, plus the sour cream, egg and other ingredients for your dough. 

Or you could just follow our Soft Cinnamon Rolls recipe or our Perfectly Pillowy Cinnamon Rolls recipe, both of which already include a tangzhong starter. 

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