i solved my Problem with this website:
On my A4988-Board is a R100 resistor and the max. Stepper-current is 1.3A . So the formula for X-stepper-servo-Vref is max. Vref = 1.3A * 8 * 0.1Ohm = 1.04 VDC for X-stepper and for the Y-stepper-servo you can double VDC, because both stepper-servos are on one A4988-Driver and share the current. Because of the fact that Y1 and Y2 share one Driver, people report, that the one stepper-servo works when they unplug the other one.
Hope my calculations are right But works for me like a charm.