Advance extrusion, extruder code help?
The extruder advance feature is probably not enabled on your printer, so this code effectively does nothing (and you don't need to mess with it). Extruder advance is a feature that tries to compensate for the delay between feeding (or retracting) the filament and the point at which it actually starts to extrude, but it's generally not used. The fact that the manufacturer left D_FILAMENT at the default of 2.85 probably means they didn't enable this. You can check whether it is enabled by seeing if there is an (uncommented) #define ADVANCE.

The reason D_FILAMENT appears twice is because they're computing the cross-sectional area of your filament, which proportional to the square of its diameter.

The arc interpolation settings have nothing to do with extruder calibration at all, but define the resolution at which G2/G3 approximate arcs. G2/G3 are currently not supported/used by most slicers, so you can safely ignore these settings since they don't do anything that would influence regular printing.

The only thing that you should change is the following line in the Configuration.h file:

#define DEFAULT_AXIS_STEPS_PER_UNIT   {80,80,4000,500}


Leave the first three values as-is (they may be different for your printer) and change the last one to 417.

You could also avoid changing the firmware at all, and use M92 E417 to set the steps per mm for your extruder, or (if you have an LCD) use the LCD to adjust the steps per mm.