Rostock Mini, problem with print dimensions

So I have had my Rostock Mini quite some time now and its works okay, but now is the first time I would need some precise dimensions for my prints. I have tried to calibrate the machine as best as I can but in the end there seems to be something weird going on as when I print 100mm by 20mm by 5mm part, the 100mm side is correct lenght but the 20mm side is only 18mm and if I change the DELTA_DIAGONAL_ROD accordingly then the 100mm side is 104mm and the 20mm side is correct.

In the end I'm stuck as I have no idea what to change in the Repetier firmware, or even where to start as it works otherwise okay.

Here are 2 ways to approach the problem:

Dimension correction via individual tower-radius
Dimension correction via individual rod lengths

So you need to identify why your dimensions are off, either your towers are not correctly positioned or your diagonal rods are not equally long.

However, both those way only works on Marlin, I have not seen/come across anything similar for Repetier.
But I guess it shouldn't be to hard to port to Repetier if one has little coding skills.