x and y Axis scaling

Hi all

Just wondering if anyone could give me some detailed advice/direction on how to scale the x and y axis in the firmware.
I found this ( [reprap.org] ) on the reprap wiki for instructions on how to do it but as I am a complete novice at this, these instructions make little to no sense.

Basically I and printing very clean straight prints however after measuring one of the prints I found it to be larger by 4%. First of all I thought this increase to be the same on all axis and therefore all I needed to do was just reduce the print size in slic3r. However after creating a print a few days ago where I needed the z axis to be exact I found it to be printing out 4% smaller than I needed.
This lead me to realise that the z axis was printing out to scale and that I cannot simply reduce the size of the print in slic3r but that what I really need to set the z and x axis at a reduced 4% scale in the firmware/coding somehow. I just don't seem to be able to find a tutorial anywhere that explains how to do this for people that have limited or no knowledge on programming.

I would be very greatfull for any help and tuition that anyone can give me in this matter.

Try this if it would make more sense:

[reprap.org]

In marlin firmware the line requiring attentiion is

Line 329
#define DEFAULT_AXIS_STEPS_PER_UNIT {78.7402,78.7402,200.0*8/3,760*1.1}

which defines how many steps are required per unit (your values may be way different compared to this example)

To simplify a lot, what ever the value is now for z axis, multiply that with 0.96 and try again. do not change other value same time...

HI and thanks for the reply, it already makes alot more sense than any forum or wiki that I have read so far, unfortunately my board as just died so I am waiting to get a replacement sent out to me. No doubt that the scaling will all be completely different when the new board arrives. Will check back then and let you know how I have got on.