During my own early BIR attempts, the most confusing aspect for me was getting the correct amount of base sauce for each curry, and at the right consistency. My advice now, to ensure repeatability, would be to think of it as splitting the finished base sauce into a predetermined number of portions. Don't worry about volume at all, just be sure to keep a thin consistency.
The problem with portioning by volume is that it depends on finished sauce thickness, which could vary from one chef to another, and even from one attempt to another. By splitting into a predetermined number of portions, you ensure that you capture the correct amount of ingredients (onions) in each portion. This can be more complicated than need be depending on what the chef states in his method. Some chefs are clear enough to state ''split into # portions''. Others might say for example, ''you should end up with about 2400 ml from which you can take portions of 400 ml''. You yourself might not end up with 2400 ml at all, and personally I find it difficult to measure that amount of hot sauce in any case, but what you would take from this example is that you should split your own finished base into 6 portions. Then there are those chefs who state ''thicken to a soup type consistency and split to portions of ### ml''; the most difficult to follow.
I'd also recommend you check the amount of meat that each chef uses with his own sauce portion, and either use that same amount of meat, or pro-rata the portion size of your base sauce to match the amount of meat you prefer to use.
In respect to sauce thickness, just think... thin is good, thick is bad. It doesn't need to be more complex than that. Obtaining exactly the chefs intended base sauce thickness is just not important. So long as the sauce is pretty thin, then it's easy to thicken up in the final curry. If you start with a thick base however, you might make the same rookie mistake as me and produce an overly thick and spicy curry. In any case, the advised curry cooking method has you flash fry the G&G, spices & onion/pepper, then gradually add base sauce to prevent sticking and burning, while importantly cooking the spices for some minutes. During these minutes, the sauce is thickening considerably and easy to control, so no harm at all in having a very thin sauce to start with. The only advantage of a thick sauce is reduced storage volume, but it should still be well thinned down before using in the final curry.
Hope it helps someone.