Every roll of filament is going to be different. even 2 rolls of the same color from the same manufacturer will be different.

These calibration steps should be done for every single roll you use the first time you use it.
by saving the profile with an appropriate name, you can then switch colors and/or materials quickly and easily by selecting the correct profile for a given roll.

I have something like 30 different profiles stored right now - one for each roll of filament I have in my arsenal.
My profile names go something like this: material type, Manufacturer ID (may be full name, may be initials) & the color
Examples:

• ABS Octave White
  ABS HB Red
  ABS Inland Green
  PLA MP Orange
  PLA SS Blue

Because I change colors frequently, this is very useful to me.

after printing one out with the above settings, use your calipers to measure the thickness of all 4 walls. Keep the angle of the caliper jaws low, you only want to grab 1 or 2 layers - not the whole wall (see green line in pic below) Be careful not get into the layer seam area as well - stay towards the center of the wall.

make note of all 4 wall measurements. Add them up, then divide by 4 to get the average wall thickness.
We are shooting for an ideal wall thickness of 0.48mm

if your wall average is higher than 0.48, we need to make an adjustment to the extrusion multiplier..

the math is:
     0.48 / your average = new extrusion multiplier

Example:
     0.48 / 0.52 = 0.92307..... 

our new extrusion multiplier would be 0.92 (round to 2 decimal places)

enter that number into the appropriate box on the "filament settings" tab of slic3r and save it.

Print another calibration cube and measure it to verify the wall thickness is correct, if not, tweak your extrusion multiplier number a little up or down to get as close as possible to the 0.48 average (if too thick, try slightly smaller (using our example number - 0.91) if too thin, try slightly higher (example: 0.93) you can take it out to 3 decimal places if needed, but it wont make that much of a difference.

also be sure to take a good look at the wall itself - once dialed in it should look something like this:

the layers are nice and straight & pretty smooth, no diagonal pattern showing (moire)
If you do have a strong diagonal pattern, it will be difficult to get good wall thickness readings.
You may need to tweak the voltage of the extruder stepper driver to reduce the moire to a less noticeable level.

Just trying to give back a little of the help I got here all those months ago, 

and sometimes pictures/screenshots can speak volumes more than mere words can.

I couldn´t have said it better myself !

i will test my settings now, after this i upload some picture of what i got..(maybe i will put all of this in a nice post together, later on )

Nice!!!
Thanks heartless

Just trying to give back a little of the help I got here all those months ago, 

and sometimes pictures/screenshots can speak volumes more than mere words can.

Open Slic3r by clicking on the configure button (latest version used here - v1.2.9)

In the "filament settings" tab, under "filament", make the following changes -

don't forget to save your changes when done.

Then, in the "printer setting" tab, under "extruder 1", make sure your nozzle size is set correctly.
0.4 nozzle should be set at 0.48
0.35 nozzle should be set at 0.42

generally speaking, nozzle diameter should be set to 120% of actual nozzle size - i believe this is to take die swell into account (?).

after making the changes (be sure to save everything) you want to select the profile you have created in RH's slicing tab, load up a calibration cube and print a single wall (1 perimeter)/zero infill version of it about 5-10mm high

you only need to print the cube tall enough to get a good measurement of wall thickness - no need to print the whole thing.

(have to head to work now...)

You can try that but I have never seen one that wasn't 105-108

mine was actually 103.something, before i upgraded to the 8825 stepper driver.

but yeah, get it as close as possible to the 100mm asked for, 100mm given - whatever that number may be.

next step will be calibrating your filament & filament extrusion width.

I hope you have a digital caliper?

measure the filament diameter in multiple places along a meter or two length (7-12 measurements)
calculate the average of those measurements and enter that number into your slicer as the filament size.

but first of all thank you so mutch with your help til here
helped me a lot