What do you have for 'batch-size' and equipment? Keep in mind you are still getting some utilization/isomerization at 15/5 but not much. So most of it will be from the 60 minute addition, so its not too far off.
I just punched it into my Beersmith and with a 4% AA Saaz I would get 24.6 IBUs by following what you gave me. Make sure your batch-size and equipment are accurate.
Primary - 1 Gal JAOM, Belgian IPA, Pumpkin ale
Secondary - 5-way split Breakfast Imperial Stout, 2 Gal Traditional mead
Bottled - Kolsch, Pliny the Younger clone, Centennial Blonde, Alpine's Nelson clone, Apfelwein, QuicknDirty Cider
On deck - EdWort's Haus Pale