...no, that's not how it works. The simple answer is that it's either worth +1 CR a level or +1 CR every two levels, up until you add as many levels as its original CR, then it's always +1 CR a level.
As Lost in Limbo said though, go to the page and follow the directions, because there's a lot more to it than just adding class levels.
That all being said, determining CR can be complicated. This one happens to be easy. You're adding Wizard levels. Wizard levels are only "key" (as in, +1 CR a level) if the monster is primarily a spellcaster (not really) and its spellcasting levels stack with its natural spellcasting (not at all). So Wizard is not key, therefore you add +1/2 CR for every level until 7 levels, then +1 CR per level. So CR 10.5 is the answer. Technically it's just CR 10, but the system doesn't include what to do if you only add one level and I feel it's helpful to include that half, otherwise you can add +1 <any nonkey class> level to literally everything for no change in CR but a massive change in stats.
As always, this should be taken with the largest grain of salt you can find, as CR is a terrible metric and adding class levels to monsters is a big part of that (the game thinks that 3 levels of Fighter are worth 6 levels of Cleric for a bruiser, despite Fighter giving lower saves, less BAB, and less HP).
