Just finished reading the build log: That.s a great library of information. Thanks for that.
Unfortunately, I did not find what I was looking for. My gantry as seen on the first link from post 218, sits on two identical 24mm aluminium plates, so no need for epoxy levelling my my case, although in the absence of a flat reference surface, this sounds the way to go.

I find straightforward the alignment of the rail as paralel and square to the other axis (4 holes test), but I have no clue how to make them coplanar. I just cut the steel tubing tiny segments that hold the one upon which the rails are mounted, as equal as I could, then when only the rails were fited I adjusted the whole thing until I did not feel any resistance, but they might stil not be complanar, as is the linear bearing have quite a tolerance. Except with a construction level, I have no clue how to check that. Then mounted the ballscrew , added the coupler and a 14mm rod on the other side, and turned it with the tips of my finger the whole distance, while adjusting the up/down amd right/left screws.
It seemed ok at the moment.

It is failry easy the release the bolts and redo the calibration, but I don.t know what I would do differently.