There's actually two different kind of flours.. One is the kind that will break off easily and there's another kind that if you mixed it and make it, it will not break easily and it's pretty thick.
i think you're talking about rice flour and tapioca starch - that's how we used to do it back in the day. and yes, you can go this route, which is cheaper if you're making a LOT - but i find it easier and more consistent using banh cuon steamed rice roll flour that's already pre-mixed with the appropriate portions.
back when we did go the former route, it was always trial and error before getting the appropriate consistency. but then again, that was also before i cared to use measuring tools. haha.