What numbers do you propose be plugged into your equation given neither any mention of an anterior open bite NOR any mention of the proposed CW rotation of the maxilla???
If OP doesn't have an anterior open bite, then the number to plug into the last term (I specified the last term as "the discrepancy between the maxillary and mandibular occlusal planes") would be zero.
Thus, in cases where the maxillary and mandibular occlusal planes are parallel, the formula simplifies to:
(CW rotation of mandibular plane) = (CW rotation of the maxilla)
OP mentioned that he is getting CW rotation, which implies the term on the right side of the equation is positive, which in turn implies that there will be "positive" CW rotation of the mandibular plane (assuming OP doesn't have an anterior open bite).
You said,
Depends on the net CW rotation. If more is removed from the back than 'dropped down' in the front, then mandible plane angle should be LESS than what you started with.
This is false. Consider a case when there is both a posterior impaction and an anterior disimpaction (downgrafting) where the magnitude of the posterior impaction is greater than the magnitude of the anterior disimpaction. Also assume OP doesn't have an anterior open bite. Since both the posterior impaction and anterior disimpaction contribute to CW, there must be a positive net CW rotation of the maxilla, which would yield a positive net CW rotation of the mandibular plane from the formula.
CW rotation of the mandibular plane is a steepening of the mandibular plane, or an increase in the mandibular plane angle. But you said that "If more is removed from the back than 'dropped down' in the front, then mandible plane angle should be LESS than what you started with." Thus, this case contradicts your statement.
@ODog, I was basically pointing out that if you don't have an anterior openbite, clockwise rotation of the maxilla will result in an INCREASE in mandibular plane angle, not a reduction like kavan said.