Hi Don,
on the attached drawing, the buoyancy tank is marked with green color, and I believe, that volume (given in technical data) 11,89 m
3 and 12,89 m
3 (at type IXC and IXC/40 respectively) refers to this green area.
However, as you noted, the upper part of the free-flooding volume (marked with grey color) also can be considered as part of buoyancy tank, because when you are blowing buoyancy tank, the joint volumes green and grey are blown.
So I think, that strictly speaking, the buoyancy tank is only the green part, but some space below (grey) can be considered as a part of it as a side effect of flooding slits located just above torpedo tubes.
One can ask, why designers did not locate these flooding slits higher, just below the buoyancy tank (i.e. between grey and green areas)? I almost sure, that it's because of the large flooding gates inside the torpedo tube recess. These large flooding gates are intended to catch any air bubbles, that can escape from the torpedo tube during the torpedo launch. These air bubbles would float toward the top of the buoyancy tank and there be stored, instead of floating to surface and betray the boat position.
Such design was developed later into
Schwallfang, used on type XXIII U-Boats.
Any other flooding slots between these "catching holes" and the bottom of buoyancy tank would corrupt this design.
--
Regards
Maciek