Bigger boil volume yields better hop utilization. That's why you're getting a larger IBU value. Otherwise, looks good, guy.
Assuming you're using 5.3gal as your boil volume, and 5gal as the final volume, I'm getting 14.5IBU also; so everything checks out in that respect. If I were going AG on this, I'd be using a boil of around 6+gal and ending around 5.5gal for an absolute final finished beer volume of about 5gal - with losses due to breaks, trub, transfer, etc. With those numbers, I would get around 13.8IBU, so even scaling the Hallertauer down to 0.5oz (due to better utilization from bigger boil volume) to give 7IBU could work, too.
I'd be impressed if someone can use their tounge to tell the difference between 8IBU and 14IBU in a brew with 3068. But you've got the right idea - just tweak everything to hit approximately what you're shooting for.