Summary

There are already a lot of z height blocks around but I was looking for maximum precision while haven also a maximum ease of use.
I found some designs leveling x to y rails which would give maximum precision, so I went for this concept. To get the ease of use I designed a one piece block which will stay stable while moving the z axis down since you will need two hands for operate the two spindles at the same time.
The process is just straight forward:

1. Disable stepper motors
2. Move the bed to the very front
3. Raise the z axis so the block fits under the beam
4. Place the block on the two rails
5. Lower the z axis manually until it sits perfectly parallel to the two lower rails
6. Use the printer control (or print server like octoprint) to raise the z axis for at least 30mm
7. Remove the block
   The design is also allowing easy printing without support.
   If you insist in two separate blocks, I also uploaded the single block design.

Updated V2: increase the tolerances you it slips easier on the extrusions...