Dual extruder with different step/mm

Hi everyone,

I am setting up a dual extruder for my i3 with totally different E step. After google and reading articles related to the topic, i come up with this:

- In configuration.h:
1. First define second extruder
#define Extrude 2

#define EXTRUDER_OFFSET_X {0.0, 0.0} // (in mm) for each extruder, offset of the hotend on the X axis
#define EXTRUDER_OFFSET_Y {0.0, 50.00} // (in mm) for each extruder, offset of the hotend on the Y axis

The extruder 2 will have the same X coordinate and offset from extruder (1) 50mm in Y axis. Is it right?

2. Set the step/mm

#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0, 0} // set the homing speeds (mm/min)

// default settings

#define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,4000,500,900} // default steps per unit for Ultimaker
#define DEFAULT_MAX_FEEDRATE {300, 300, 5, 25, 50} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {3000,3000,100,10000, 10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION 3000 // X, Y, Z and E acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION 3000 // E acceleration in mm/s^2 for retracts
#define DEFAULT_TRAVEL_ACCELERATION 3000 // X, Y, Z acceleration in mm/s^2 for travel (non printing) moves

// The speed change that does not require acceleration (i.e. the software might assume it can be done instantaneously)
#define DEFAULT_XYJERK 20.0 // (mm/sec)
#define DEFAULT_ZJERK 0.4 // (mm/sec)
#define DEFAULT_EJERK 5.0 // (mm/sec)

*** The number is just for example.

- And refer from [github.com]
in planner.ccp when using the variable axis_steps_per_unit the use with the extruder active.
target [E_AXIS] = lround (e*axis_steps_per_unit [active_extruder + 3]);
The + 3 to increase the value of active extruder of the three previous axis. I DONT KNOW EXACTLY WHAT IS THIS MEAN?

Is there any thing else to modified?

Thank you for your time.