The extruder steps/mm value has to be calibrated precisely and should be tuned at low print speeds (20mm/s) to elimiate nozzle pressure effects
Calibration is needed - Print some test cubes, each with another K value. You can set the value with “M900 K..” before you start the print, where .. is the value you want it to be. Start with K0 to disable the pressure control, which will give you a reference print how your cube would look like without the feature enabled. Then increase K maybe in 25 units steps. On every print note down the used K value.
http://marlinfw.org/docs/features/lin_advance.html
But