My apologies. As you astutely pointed out, I do in general prefer intuitive mathematics. I am also aware that there are times when the simplified version of an equation loses its transparency. Perhaps that is what has happened in this instance. As you suggest, I certainly have not read the references you provide.
I understand the change in effective aperture, and apologize for my denseness in not recognizing it as you stated it originally. I had never considered it as a factor with enlargements, but I can see how it would apply. I am interested a bit because I've used my intuitive approach for some time and have never had to recalculate exposure, perhaps I ought to go back and compare some of my prints of different sizes for tonal variation.
On the other hand, perhaps I won't, since it seems to be working...