I have a new bookcase (read garage put together shelving 2100 high by 1800 wide by 400 deep). I thought I'd put books on it A-F, G-L, M-R and then S-Z. Surely they'd average out to roughly the same volume per shelf...well maybe not the M-R shelf - how bad could it be though.
So far, with about 5 boxes of books to go, A-F is 2.5 layers deep, while G-L is only 1 1/3.
M - R is just under 2 deep, but I have yet to unpack the 2 boxes labeled McCaffrey and and Pratchett. I forsee having to put 1/2 of M on the shelf above and QR on the shelf below.
Then I expect I'll be considering buying another shelf. If I adjust them all I can fit one more in. I hope the house supports can take the load :)