Andrew R Stewart 

There's no way I would put that out on a bike I or others would ride. I take it that the "recessed" nut is bottomed out on the front side of the steerer. This offers the caliper much less resistance to the rocking that braking action can create. When the recessed nut is properly bottomed on the crown's backside there is a significantly greater bracing ability, the one designed and intended by people far smarter then you and I.

Now when one adds a bracket to the mix the importance of keeping the caliper's mounting bolt tight and positively held is even greater.

So what to do? The best option is to get a shorter nut. One that will have it's shoulder seat against the crown's backside. It's possible that the current, too long, nut has enough threads to just cut down to an appropriate length. But most every shop I've worked in has a lot of these nuts bin various lengths. Some bikes come with more then one and the extras get tossed into the misc. bin. Andy