The slicing software knows exactly (given accurate E-step, filament diameter, nozzle diameter values) how much cold plastic is pushed in the extruder and how much mass is extruded.
Why should the extrusion multiplier be changed from 100%? Shouldn't the slicer extrude already enough to fill the gaps left by the neighbouring perimeter (see rectangular vs recto-oval extrusion profile).
As result of the "why" question, implicitly comes up the question "how" should I (scientifically, not try and fail) define the extrusion multiplier? it would appear that said value should not be touched if the rest is done properly.
The only idea I can have is to print at 100% infill and then saw the part, to check how big are the voids between 4 neighbouring extrusions. Measuring the wall thickness seems very prone to errors (but again, is such calibration really needed?)