Now I’ll show you how and why I used these exact doses, i.e., the percentage of “water roux” to use in doughs (even those made with traditional flours) and the doses of starch and water needed to make the so-called water roux. Since this is a gluten-free recipe, I will focus exclusively on the doses provided by the Tang Zhong for a water roux made with pure starches like potato starch or cornstarch (as in this case).
a) For a cornstarch water roux: the ratio between cornstarch and water must be 1:10 – that is, for every gram of cornstarch, we will add 10 grams of water.
b) The percentage of water roux to include in the dough is 30% of the weight of the flour used in the recipe.
c) Practically, using the reference doses from this recipe, this means that 30% of 250 grams is 75 grams (i.e., the water roux should weigh 75 grams). Finally, to understand how much starch and water are needed to reach 75, divide 75:11 (the sum of 1+10 from the starch-to-water ratio) and you will get 7 grams of cornstarch (rounded up) and 70 ml of water (also rounded up). Obviously, the total is not 75 but 77 grams, but the result will not be compromised; the important thing is to keep the 1 to 10 ratio intact.

If the amount of flour used in the dough had been 500 grams, then the water roux would have been 150 grams; referring to the calculations I indicated, you can adjust accordingly each time for the amount of starch and water.
I wrote all of this not to give you a headache, but for the sake of precision. However, don’t worry because by following the doses I indicated, everything will be easier.