How do I calculate actual IBUs?

I do partial boil extract recipes and I'm trying to figure out how to make an IPA with a lot of hops. I've read a lot of the other threads on partial boils and maximum IBUs, and I have a good grasp on how high you can go.

I have been playing around with Beersmith, and the IBU calculator is obviously a bit odd. It lets you add hops and never stops increasing the IBUs. I got some good info from this thread:

https://www.homebrewtalk.com/f14/attention-all-partial-boil-brewers-91059/

but without going into a lot of math and averaging Tinseth estimation with other estimations, etc. etc., here's what I was thinking:

What about just adding hops to my Beersmith recipe until it shows 100 IBUs, and then do some easy math those IBUs with the 0 IBUs from the top off water.

For example: 3.5 gal boil with 100 IBUs, 1.5 gal top-off water (0 IBUs) would make a batch with 70 IBUs. (3.5*100 + 1.5*0)/5

Is this close enough, or am I missing something about Beersmith?

You've got it right. A somewhat simpler equation and the one I use is, (Vb/Vt)*IBU where Vb is boil volume, Vt is total batch volume and IBU is the IBUs BeerSmith calculates for you.

If you want some extra math, try the calculation at the very bottom of this page. It might be a way to doublecheck BeerSmith.

Brewing Formulas

OOPS, sorry its not the bottom, its kind of in the middle. You'll see it, its a section titled IBU.