Installed the master rail:

See the clamps bowing? I have it as tight up against that lip as i can.

Now here is how I tested the height:


Dragged the indicator stuck on the 2x4x6 block, and slide the block up and down. Absolutely ZERO movement on the indicator... as I will show here:

the indicator isn't really in focus (damn cell cams) BUT, you can see the indicator not moving at all, until I slide the block out from under the indicator.