it can vary depending on the system, but my guess is that it is simply the geometric area that the shear force is acting over. For a rectangle simply base*height. If this isn't it then describe the situation a bit more.
Be careful in how you compute the bending and shear stresses! The standard BendingStress = Mc/I equation won't be very accurate considering the low aspect ratio of the beam. The ShearStress = VQ/junk will also be not as accurate. This case isn't too far off from a bearing type stress either.
Is saying that the ultimate shear allowable is approx. equal to 0.6 times the ultimate tensile allowable if only the tensile allowable is known? I'm trying to "fix" a horribly made appendix and I've never encountered this relation before.