I got pretty good results with Prusament PETG after the manual calibration in Softfever, same goes for Devil Design PETG.
I started with the Generic PETG profile and changed these settings:
So I basically changed flow rate, pressure advance and volumetric speed according to the calibration results. With the Prusament I turned down cooling a little which was not needed for the Devil Design PETG.
And yes, I get decent quality with 22mm³/s.
But I can confirm that there is quite some oozing which might affect first layers. Using PLA there is no oozing at all.