ORIGINAL: HighPlains

The frontal area of the wing does not matter one bit. You do take the area and multiply by the Cd. For fuselage, it is the wetted area. However, if you assume that part of the flow on the fuselage is laminar, then that portion has a different number than the part with turbulent flow.

At one time it was thought that it was not possible to have laminar flow on a fuselage with a tractor engine, but later studies showed that between pulses of turbulant air from the prop, that the flow could revert to laminar.

I found the same thing working with water cooled power transistors, it is very hard to maintain turbulant flow in the water channel. While you want laminar flow on aircraft, it is a very bad thing if you want to transfer heat.

The interference drag between the wing and fuselage, or the fuselage and tail surfaces is generally assumed to be between 5 to 10% of the combined wing and fuselage drag, and the tail interference to add another 10% of the tail surface drag. All a guess. Cooling drag is usually significant too, with most light planes rather dirty. That can add 10 to 15% more drag to a design. Even air leaks into the airplane's cockpit adds drag.