Rear corner dip

I've been wrestling with this for hours now, after having installed the Enclosure kit earlier today - each adjustment I make seems to throw something out of whack. I'm sure I can improve on this since the tolerance before installing the enclosure was about .19mm. I have been unscrewing the left and right extrusions and adjusting them up and down to try and bring it into square as detailed in the FAQ video, and measuring my adjustments with a caliper just to make sure I'm not going mad - any ideas appreciated! Thank you,