IBU not displaying accuratly

So I enetered my first recipe into BeerSmith. It's a 5 gallon batch with a 2.5 gallon boil. The hop additions are as follows.

3/4 oz Challenger @ 60 min
1/2 oz Challenger @ 15 min
1/4 oz Williamette @ 15 min
1/2 oz Williamette @ 3 min

My recipe states this is 26 IBU, but BeerSmith says 16 IBU.  Is there somewhere I can enter my boil Gravity? That would give me an accurate IBU level right?

Any ideas?

Robert

Robert,
 If you entered the other ingredients (grains, extracts, etc) it will automatically calculate your boil gravity and use that in the IBU calculations.

 Most likely the problem has to do with the IBU forumla used.  BeerSmith uses Tinseth by default, while many other programs use Rager (which reads higher).

 See the following link for a detailed explanation.
  http://www.beersmith.com/bitterness_(ibus)_too_low.htm

Cheers!
Brad