I've been playing some more and comparing the results to books off the shelf. I finally decided that 11pt Georgia was a little too big (made it look like a book for younger readers), so I dropped to 10.5pt but kept the line spacing the same (I'd already been using a slightly larger spacing). The result is actually quite good I think - though I guess the proof will be in the ... um, proof, I guess.
It's amazing how much time you can spend on the minutiae trying to get it look right. Does it get any easier as you go on?