Help with initial mesh setup

hey folks i´m building a vcore 4 500 with idex. tried to tab down the back corners which helped a little. the warping along x axis did not dissapear. saved the config, homed the machine and did a z-tilt routine. after that the mesh scan looks like in the beginning. i have a machine with a back plate mounted already. should i unbolt that to get to the screws on the back of the frame? however, there is a middle post holding the whole back secion up. in order to lower the frame it would be necessary to cut this one shorter. should i worry or is this whithin normal tolerances? also irritated by the dimensions on the top right. it says 40x40. what does that mean? my machine is 500x500 thanks for your help