Saw your ninja-edit change of tone.

Still, EXP obtained would let us know which level of BB you rolled.
You're right that Shell II -> IV is just <8% MDR difference, barely more than if you had a D.ring, and less than if you had M. ring too.
I'm still surprised Firaga 3 did so little. With a gimp stoneskin set (55+24 mnd, 105+5 skill = 171 HP), 296 + 171 = 467 damage taken. You have 19% MDR plus 21.88% from shell, giving a post-MDB Firaga 3 of only 790 damage. Assuming you didn't have any lamia +1, that's a base damage of only 885. Don't normal lanterns nuke harder than that?