steps of my xaxis are not correct

I have the suspicion that the steps of my xaxis are not correct. In the ydirection, radii print quite usefully, in the xdirection they become oval. I have already varied the belt tension and found no changes. Are the steps of the xaxis adjusted in the same way as with the extruder...so can I also use the EStep Calculator or do I have to proceed differently?

@axiom I would start with the calculated value based on motor steps/turn , microsteps and pulley teeth.
Depending on the type of printer (Delta or CoreXY) you also have to know some parameters for the kinematic model. 
xy_steps_per_mm = (motor_steps_per_rev ∗ driver_microstep) / (belt_pitch ∗ pulley_number_of_teeth)

@axiom said in steps of my xaxis are not correct:
,...............Are the steps of the xaxis adjusted in the same way as with the extruder..........
No. Extruders are different because there are variables such as how far the teeth that grip the filament bight into it. With belts and pulleys, no such variables normally exist, so the calculated value based on known dimensions should be correct.