Once I had the fwd CMM held in place with bog I removed my timber frame and cut it down so I could use it to level the Aft CMM.
I put straight edges though the hull and used a tape to set the spacing. Then I braced the position of the Aft CMM by clamping a length of alloy box section two the two CMMs.
A friend brought around his engineers level to check my work leveling the beam pads. This level is extremely sensitive and it worked out my greatest error was only 0.35mm.
This was basically 3 thicknesses of peel ply under one end to make it perfectly level on the worst beam landing. I was very surprised it worked out so well.
A few more pics below.