Well that's reversing the system, when I'm advancing a monster I don't add 2 class levels to raise its CR up by 1, since that's ridiculous because classes are vastly different and therefore can't be reduced to such an arbitrary system. Instead, I add whatever number of class levels I'm looking for and then calculate the CR afterwards just to get a rough estimate of what the rewards should be.

Not that there's anything wrong with your interpretation, the halves are rounded down anyway so it doesn't change the en result. Still, it needed to be pointed out since it wasn't clear what you were doing in your previous post.