This is going well... (should I have said that?)
The bed control software is working well , I've a couple of incidental hardware issues to fix .
Today i decided to align the bed : first the rails were trammed using a inclinometer (aka gunners quadrant) - these super accurate adjustable levels , it can easily detect a piece of paper (3 thou") inserted under one end of the bar shown , I'm confident I got the two rails planar to better than 50um.
Tramming the bed-frame was a fiddle , I could level the two front ball-screws using the toolhead mounted DTI , I had to use the Inclinometer for the rear one.
leveling the bed segments was (as mentioned above) a right PITA . I had to trim the tabs of a couple of panels to get them to sit flat with the meeting panels before I could adjust it. Once all the panels were close to zero , so there was a little less interaction between the adjusters ,it was possible to get it flat to better than +/-50um .
If I were to do this again , I'd make the plate from a single sheet and just cut thermal relief slots between segments.