Don't bother about it, it's less than 100mm :whistling:

No seriously you will always have some errors on a build like a gantry, the answer is to machine your sacrificial board in situ to flatten it.

Someone put a program on here I think to do just this [ or may have been on the Vectric forum ] you just plug in max travels and depth of cut and it goes all over the bed and takes a shaving cut.

[Edit]
Or use the surfacing wizard in Mach.