Hey, cool. I'm trying to nail down how much bitterness comes from mash hopping. You say it's very bitter...... how bitter is that? Like 100 IBUs bitter, or more like 50 IBUs bitter? If the former then it may be safe to say that a lot of isomerization happens in the mash and carries into the boil. If the latter then maybe it's more like partial isomerization which continues in the boil but to a much lesser extent than all of it, and might be equated by saying it gives the same IBUs as about a 10-minute boil addition, which is what is assumed in my chosen homebrewing software (StrangeBrew).
So... can you guess where the IBUs actually taste? Is it terribly bitter like a double IPA, or is it just "very" bitter more like a pale ale would be?