How do you calibrate your x,y and z?
For what? The steps should not need any tuning, it is a fixed number. What do you think is off?
Oh? So what if one of your axis is off just a tad?
It should not be, how are you checking, and what is a tad (10mm or 0.001mm")?
I have read online that if your belts streach or you retighten them you should recali rate? Just curious. How you did that in marlin.
The belts are fiber lined, if they change you should replace them. They are only rubber on the outside, they are belts inside. There is a lot of mis-information out there.
To change it you change the steps /mm in the firmware.