VC4 500 IDEX initial cold mesh

Good evening, today I reached the point where I could do my first mesh. On the first try, I had a 0.600 mm range. After re-tightening the linear rails and pulling the X-axis, I achieved a 0.350 mm range. The X-axis bend is around 0.2 mm. Afterwards i tried to fix the "hanging" back of the bed, but somehow i can't get under 0.35 mm. If I push the back right corner, the left right corner lifts and the other way round. Because of the vertical extrusion in the mid back, it acts like a rocker. I also tried slightly bending the horizontal extrusion in the back downwards, but no success. Do you have any suggestions how I can improve this mesh?