Extruder step rate in Marlin

Hello,
I'm new with 3D printing and I have a reprap compatible printer. I've read in the slic3r documentation that "Extruder is calibrated in the firmware with the
correct steps per mm of filament" and that "the extruder step rate is vital."

Is it editable? If so, where can I find this instruction in Marlin?

I found this firmware guide (http://solidutopia.com/marlin-firmware-user-guide-basic/) and I browse through all of it but it seems to have no reference to that.

I've got Arduino IDE to edit the firmware. Thanks in advance for any help

It's the M92 command to set steps/mm.
You can add a line like " M92 Exx.xx" in the startcode of your slicer or change the value in marlins configuration.h

Line 479ish example:

#define DEFAULT_AXIS_STEPS_PER_UNIT   {80.0,80.0, 4000.0, 93.0} // 1/32 for XYZ and 1/16 for MK8-extruder

If you don't have a dedicated marlin version working for your printer at hand, I would use the start-code method to change a single value.
-Olaf

Edited 1 time(s). Last edit at 11/04/2015 04:04AM by o_lampe.

I'll prefer to change the value in Marlin, for now. So the 4 values within brackets should refer to X, Y, Z, E axis and that of my interest should be the last one - is it right?
If I double it, I should have extruder step rate doubled... I'll try tomorrow.
Many thanks for your help!

Edited 1 time(s). Last edit at 11/04/2015 04:17PM by ramne.

I've tried and it works!
On the contrary of what I said, the command line is easier to make a test.