I would add a plane parallel to the component B and use the offset dimension of Dimension between component B & C, Divided by 2 (since you should know the distance) so that the new plane is between components B and C.
The sketch approach is good. Don't forget the measurement variable method. Measure between two planes, save the value as a variable. The dialog to do so is available in the measurement ribbon. Use that variable divided by 2 in the formula for the plane offset. Thereafter, as with a sketch, the values will auto-adjust if you move one or both of the components.