Extrusion Calibration

I'm attempting to calibrate the extrusion amount but getting some unexpected results. I have calibrated the feed per step and measured the nozzle diameter (0.41 mm). My filament seems very consistent at (1.75 +/- 0.02) mm. I created a design with wall thickness that vary from 0.3 mm to 1.8 mm. I had initially calibrated the extrusion to the "external perimeters extrusion width." For any design wall thickness < 0.8 mm, Slic3r seems to use the external perimeter width, which is fine. But then for slightly larger wall thickness, it seems to over extrude. For very large wall thickness, the actual wall thickness approaches the design. Is this behavoir normal or are there some settings that I can tweak? Printer: Creality CR-10 V2 Slicer: Slic3r