Please excuse the interruption, since I haven't been closely following this discussion. I hope the following isn't overly repetitious.
I have no qualms with whatever internally consistent math scheme you want to use, and when I've played around with your "absorption" term in place of the "water retained ratio," I found it did in fact simplify the algebra.
But I really don't care for the name you are using, "absorption." That word has a common, everyday meaning which is understood by every lay person: the amount of water absorbed by the grounds. You use it differently (incorrectly, most people would say) since -- as I understand it -- your "A" is really absorption minus extraction.
The difference between the two isn't huge, but it's a key difference: the whole science of making coffee hinges on that extraction term. We are trying to educate people about how the extraction process works; you are, too, but you're being sloppy and/or stubborn (IMHO) with your terminology.
In technical education I think proper terminology matters. Being sloppy does not advance the cause. I'm not sure what you should be calling your "A" term, but absorption isn't it.
I take it at face value: 2 grams of water absorbed per gram of coffee. It's fascinating that we see it differently.
Your approach includes an additional, albeit negligible, mass transfer. The formula works great. I'd use it. But A isn't really the absorption (ratio). It's something else. To illustrate, suppose instead of 2 g/g the water retention for coffee was a lot less, say, 0.1 g/g. Then for a 20% extraction yield we'd get this:
LRR = 0.1 g/g A = -0.1
LRR still means what it says: 0.1 g of water absorbed per gram of coffee. But a negative value for the so-called "absorption"? What does that mean?
If it were me I'd come up with some boring term like brew coefficient and continue using A or K or whatever letter fits.
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