I would love to see more control over when the fermentables are added (honey, sugar, etc) as well as which are not fermentable (don't add to the AA%.)
To work it out, though, just set up your recipe, then make your extract amounts "zero." Note the IBU calculations for the first part of the boil (before the extract goes in.) Add back your extract amounts, and add the first IBU numbers to those showing after you put the extract in.
IOW, If you have a 60m, 20m, and 5m addition, and you add extract for the last 10m, make your extract "zero" and note that at 60m you have 30 IBU's Challenger, and at 20m you have 10 IBU's Cascade. Put the extract amount back, and ignore the first two IBU's and add to the "40" however many 5m IBU's you have, in this case 4 IBU's of Cascade. Consider your IBU's 44, even though BS says 32. Down in the notes section, record it till Brad gets a fix. (HINT!)
