Hardbound book spine sizes:
Up to 80 page books = 0.375”
Add 0.125" per extra 40 pages (20 sheets of paper)

Softbound book spine sizes:
Starts at 0.25" for 30 pages
Add 0.125" per extra 40 pages.

Layflat book spine sizes:
Thin pages start at 0.5"
Rigid pages start at 0.75"

Add 0.25" per 7 extra spreads on thin.
Add 0.25" per 5 extra spreads on rigid.