I had my backing put on at the fly shop that I frequent. They followed the manufacturere's recommendations and put on enough of it that between the backing and the fly line, it pretty much filled up the reel.

The problem was that when I was taking in line, sometimes the fly line would "stack up" in one spot, making it thicker than usual, and then the fly line would rub and drag on the reel. So, I took a bit of the backing off. Now, I've got some extra space, but I don't have the rubbing issue.

So, you might want to consider using a little less backing than recommended.