Your numbers are correct: I wish I agreed entirely with your analysis, and I stress I very much hope you’re right in it!
My perception is a little more negative, however: California is key, and it’s (a) the biggest state in terms of delegate numbers (b) winner-take-all and (c) strongly favorable to Romney, as things stand.
If he DOES win California’s 172 delegates, he needs only 39% of the remaining delegates from all other outstanding contests, to take the nomination outright: doable, unfortunately, especially with the conservative vote as split as it is.
If, on the other hand, he DOESN’T take CA (irrespective of who else does), he then needs 55% of all other remaining delegates to take the nomination outright: which is a tall order and one he’s unlikely to meet.