With the top of the 442mm 2040 tight we need to do the same thing to the bottom.

The blue line should be about 20.2mm. Once your gap is set at the bottom of the 496mm 2040 snug the 4x m6x16mm screws tight enough so the 2040 doesn't move.

Set the gap for the purple line to 178.8mm and tighten the 2x M6x16mm for the 442mm 2040 on the bottom face of the 496mm 2040.

Double check the measurements on the bottom(blue) are still about 20.2mm on both sides. Once both are equal tighten the 4x M6x16mm for the bottom 496mm 2040 so they're fully tight.