The way we'd been playing it, if I had cast a Weapon:5 spell, rolled a 1, then took the backlash, we treated the attack roll as a 5. We did this on the basis of, the wizard was paying the price to "force" the spell to do what he wanted (i.e., to hit what he's aiming at), so he'd get the boost.
What I meant by that example was, if I cast a Weapon:5 spell and roll a 1, there's no incentive to take backlash -- just about anything out there worth hitting with a Weapon:5 attack is going to dodge a 1, meaning it's taking an extra hit for no real benefit.
It honestly rarely made practical difference in my games. My players tended to make sure they had aspects to tag or fate points to spend anyway. When they did have to take backlash, it was usually 1 or 2 shifts and even with this clarification in play, that just meant their target had to try to dodge a 12 instead of a 14 (things get a little nuts in a 16-refresh game). Of course, the badguys had the rule too, which helped keep them challenging (for whatever reason, whether I'm using physical dice or an IRC bot, my villains tend to roll terribly in climactic fights).