When I take spells out, I put them in a little pile. Then I either (a) make a pile of spells I want to put in, and compare the sbp's of the two piles, or (b) cancel out an equal number of sbp's from the outgoing and incoming spells. But when I (think I) am done, I still re-count the whole spell book, and sometimes discover I'm out by a point or two due to making a mistake. But I try to only do this count at the end, rather than over and over with each addition/removal.