All I know is that extra tapioca flour turns out a softer product in my experience, (especailly for my tortillas). I often "up" that a bit.

I usually use only brown rice, potato starch and tapioca. Occasionally, sweet rice flour.

I've seen great variations in density of flours based upon brands. I find the oriental white rice flour to be much much denser than Bob's Red Mill. I've found Swan's potato starch to be much denser than Bob's Red Mill. Perhaps sifting would make a difference. Who still sifts flour?

In any case, I have found flour can be a huge variable, but...I agree that recipes can be rather forgiving as well in that you don't need exactly the same flours/ amounts as called for. It really is about experimentation. If at first you don't succeed...try, try again!

Cara