When you formulate the recipe these ammonium bi carbonate, sodium bi carbonate and some acidulants are added. When you see the Dough pH for hard dough variants it should not cross 6.5-7 that is thumb rule suggestion I would say. But you need to hit this with trials runs depending upon the final gauge and biscuit weight you are designing.
It is very difficult to give just like that suggestions on percentage or quantity based on flour or full wet weight recipe.
Yes there are guidelines. Commonly used leavening agent which are normally used in semi hard sweet dough is Ammonium Bicarbonate. its % differs product to product by hit and trial method. You can share your product with with me i will guide you further in this regard but without knowing actual product/issue/process it is difficult to guide in this regards.