Not 85%?

Still trying to get my head round milling definitions.

I’ve been milling grain, then putting it through two sieves (three in this case because there’s some chaff in the mix) which leaves me the chaff, the bran, the middlings and the sifted flour. To achieve the blend I want I’ve been adding 15% (of the weight of the sifted flour) bran to the sifted flour and calling it 85% extraction. And feeling very uncertain about whether this is correct or not.

Clockwise from top left, chaff, bran, sifted, middlings.

For example (disregarding the chaff which shouldn’t have been there in the first place) this morning I started with about 895g grain. After milling and sieving I end up with:
Bran – 286g
Middlings – 44g
Sifted – 560g
Milled Total – 890g

85% of 890g is 757g.

15% of the sifted flour is 84g. This much bran added to the sifted flour makes 644g which is only 72.4% of the total milled weight.

So, is it correct to describe this blend as 72.4% extraction?

That’s only the start. What I’m doing is using two sieves, a 60 mesh and an 80 mesh (at least we are now clear that the smaller the mesh number the bigger the holes in the mesh). So I class everything that doesn’t go through the 60 mesh sieve as bran, and what doesn’t go through the 80 mesh as middlings.

Is this correct? Are their standard ways of determining what constitutes bran and what middlings?

I mean, I could just say it doesn’t matter. I like the blend I’m making and, in this context, “correct” is neither here or there. But I do like to know where I am …