I promised to publish some infos regarding the levelling of the aluminium bed. @Bigchepin
I think it needn’t to be commented 
… but to be honest I was really surprised by myself to get these results. Only in the right angle of the bed I get 5/100mm tolerance. On the other steps exactly 0!! 
That’s OK!
On the Z-axis I get tolerances of 1/100mm per 1cm height.
Here I measured the height of 10cm and I got 10/100mm tolerance. That’s OK from my prospective.
And for the squaring I have designed a new tool to adjust the autosquaring feature of my controlling unit.
I printed a tool to fix the pipes of the x- and y-axis to adjust the inductive switches.
The tool can be exactly adjusted with thread rods (4 per tool). In this way you can exactly adjust both tools to get the pipes in parallel.
To place it in the position without moving I used the stopper to fix the truck completely! (shown in the foto above)
Placing the tool on both ends of the axis you can definitly position the end switches (in my case the inductive switches).
Repeating it on the y-axis you get the switches exactly positioned to use the autosquaring funcs of the controller. It works really well!!