For my second batch of homebrew I'm making a Belgian Tripel from a kit. The kit recipe calls for a 2.5 gallon boil, but I have the capacity to boil the full batch if desired. I played with the hops calculator in my demo version of Beer Alchemy to figure out the hops adjustment for a full boil, but it isn't making sense to me. The kit describes the OG as 1.083-1.086 and IBU 24-30. The hops in the kit are 2 oz of "Kent Bittering Hops" (4.9% AAU) to go in at 60 min, and 0.5 oz of "Kent Aroma Hops" (4.6% AAU) to go in at 15 minutes.
If I plug into BA wort gravity of 1.085 (is this the right number to use for wort gravity?), a 6 gallon boil and the hops mentioned, assuming pellets in a bag and Tinseth calculations, I end up with an IBU of only 24 - barely into the style range. Doesn't seem right considering these are all the hops used for a smaller volume boil. Also, how could I work backwards to see how much bitterness the hops add to the original 2.5 gallon recipe without knowing what the gravity of the 2.5 gallons is? The big question, though - How do I calculate hops adjustments when going from partial boil to full boil?
Also, the kit contains an 11g packet of Nottingham yeast. This seems like the wrong kind of yeast to me. Should I substitute a Trappist yeast instead? And what adjustment should I make to fermentation if I switch yeast?