If you are replacing a pair of stock brakes, the safe route is to reuse the nut that came with the bike, since it will be made to work specifically with this fork.

If you don’t have the stock nut, you need to make sure that you have enough thread engagement between the nut and the brake anchor post. You should have at least 10mm of thread engagement or “overlap” between the nut and the brake anchor post to ensure that your brake is safe.