Having a common point to adjust both YGantry’s rotation around Z (starting from the 100mm requirement from the back feet to the ZPlate and adjusting until XRods are parallel with bed grid) and also YGantry’s rotation around Y (actually rotating the ZPlate by having one YRod bottomed in the ZPlate’s slot while the other is adjusted so that the bed is flat/parallel to the XRods) is a pita.
When you adjust the nuts below the bed you loose some of the fine adjustments you did by raising the YRod from the bottomed position and viceversa.
This printer might be convinced to print OK prints quality-wise and/or even tolerance-wise (via software compensation) but to build it square and all things straight and parallel/perpendicular is a nightmare and I am not convinced it can be done.
As @Stephanie_A sais, YGantry twist is a bitch. Now this seems to be the most I can get it to and I get to choose which section of the bed I will make square to the ZPlate.
Ok, did my best to square bed grid to X Axis (ZPlate face) and then turned the printer its normal position for the bed leveling (making the ZPlate higer on one corner) but before I attempt that (which might screw the other adjustments as they are done from the same point which is the under-bed nuts) I did a couple measurements and played a bit with http://plot.ly (if you know a better online surface plotter pls let me know):
https://plot.ly/create/?fid=plotlythor:1
(PS: you can rotate the map with your mouse)