So in other words, what I'm getting out of finishing hops isn't bittering anything, it's all flavor and aroma? I might have known that and then forgotten it. It's been a little while since I threw some wort in a fermenter.
You'll still get some bitterness for the late additions, but the %AA differences will have less effect on changing the total bitterness levels. Think of it this way -- Adding only 2/3 the flavor hops because the AA is higher will greatly lower the flavor. Think of flavor and aroma hops as constant with the weight. Use the Bittering addition to adjust total bitterness to compensate.
Edit: Okay, so I stuck the recipe into Beersmith, but the gravity isn't coming out right at all, and that's probably gonna throw the IBU's off, isn't it?
For a BIG beer (>1.070), yes, by a lot. For a medium sized beer(around 1.050), a bit. For a Small beer, not very much.
Edit2: So I ended up using 1.3 ounces of the bittering hops and keeping the late hop additions at 1.5 ounces. We'll see how it turns out.
Good deal. That sounds about right.