If you want to maximize flavor or aroma without going overboard to where you are wasting hops (point of diminishing returns), you need a maximum of 0.8 oz/gallon (or I guess that's 23 grams/gallon).

If you want a moderate amount of flavor or aroma, use 25-50% as much.

To calculate IBUs, including the same late additions:

IBU = oz * AA% * [sqrt(4.6*Boiltime)/V + sqrt(HStime)/V]

where

V is post-boil volume in gallons,

Boiltime is in minutes, and

HStime is post-boil hop stand (or whirlpool) time in minutes that it hangs around between about 150-190 F.

So in your case, by my calculations, you'll end up with about 45 IBUs, a little higher than you had calculated but we're both in about the right ballpark, someplace between 35-45 is what you'll end up with.

Yes, that's my method, based on... well, I could tell you, but it's kind of a mess. Bottom line: It's my method.

Cheers.

EDIT: When I double-checked my math, I got 36. Now I'm gonna have to triple-check and figure out where I screwed up. But anyway. Yeah.

EDIT2: Yeah, it's 36. The + sign in my formula is no longer actually a +. I've changed it to an either/or thing, depending on whether boiled or whirlpooled (hop stand). Easier this way. Should look something more like:

IBU = oz * AA% * [sqrt(4.6*Boiltime)/V **or** sqrt(HStime)/V]

I'm pretty satisfied with this version, don't think I'll tweak it much anymore. It works for typical worts between about 1.040 and 1.080. Above and below those points, change the 4.6 to like 5.2 or 4.0 to still arrive in the right ballpark. This is an estimation method, not an exact answer, but will match reality within a few points every time as long as your OG isn't crazy high or low. I dare anyone who is interested to try it out on a few recipes and compare vs. Tinseth (at least for boil additions, there is no Tinseth for whirlpool additions).

Anyway...... I hope this helps some soul out there.