Dealing with different ibu's

Im brewing Ed's famous haus pale, and noticed his cascade hops are at 6.6%. Mine with age figured in through beersmith are at only 5%. Where should the loss be made up, all in bittering, or evenly spread out through all the additions?

Additions are @60-30-15-5min.

Through Beersmith at 5%AA for the Cascades I show a drop from 36.3 to 27.5. I would just up it a little bit throughout.

I think there are a couple of factors involved, so I don't think I would be too worried about it.

Across the board would probably be the most correct approach, easiest would be in the bittering addition. However, iirc as david_42 has mentioned in the past, it is very difficult for most people to distinguish between 30 and 40 IBU (I think it was a 10 IBU difference, david correct me if I am remembering it wrong), let alone 30 and 32 or 33. If the AAU value is low just add a few grams more in the bittering addition.

Your best bet is to make it up at the bittering add. Increase that add by 1/3. (6.6/5 = 1.32)