Need to calibrate my Z axis

I've a prusa i3 rework with RAMPS 1.4 and Marlin firmware.

Today I've completed the extruder and x-y-z axis calibrations.

So I tried to print a 10mm cube and this is the result:


X and Y axis both are 12mm (quite good). Instead the Z axis is too much compressed: 6mm . While it was printing the extruder seemed too pressed on the filament.
I currently have the following define:
#define DEFAULT_AXIS_STEPS_PER_UNIT {100, 100, 2560, 785}
where 2560 came from a default value.
I don't know how to setup the Z axis: on my Z axis I'm able to insert a M5 nut correctly.

To calibrate z make a mark using a marker pen on your smooth rod where your z carriage is located. Then command it to move by an amount such as 50mm. Measure how far it moved. If it moved 20% too far then reduce z steps per mm by 20% send the printer m92 z(new value). Then do it again check it moves the right amount. Once correct then reflash your firmware. If you have eeprom enabled you can just save setting by sending m500, saves time and to avoid having to reflash. 2560 steps per mm if you are using threaded m5 rods sound too low mine was 4000. Indeed 2560/4000 =0.64 so a 10mm cube would be 6.4mm. Sounds like this is your problem.

Your extrusion looks all over the place too, but get the mechanical calibration done first then move on to the extruder the hardest part to calibrate well. Use ' triffid hunter's calibration guide " just Google it. Its explains very well how to do it.

Good luck.

Any luck with this?