Building a P3Steel,
Getting ready to set the Z axis threaded rods and tighten them in the flex coupler of the Z motors.
What is the best way to "level" the whole X axis assembly in relation to the rest of the printer?

Thanks,
Slack

I don't know what is the best but the easiest for the average person is to build on a flat level surface checking that your Y rods are level and that your print surface is flat and level. Then just use your level to check that the Z rods are vertical (most levels measure vertical also) and then check that the X rods are level. Keeping everything flat, level and perpendicular to each other can be a real challenge but it pays off in the end. Good luck and I hope this helped.

You could use a digital caliper to measure from bottom of the smooth rod to printer frame, or desktop. Just make sure everything is nice and flat and sitting straight. All bolts and nuts tightened and thread locked.
You could also cut flat headed piece of rod and use it as jig to get both sides at the same height.

I'd use the jig and mark the correct position on the couplings/motor for easier, visual adjustment later. It might go out of sync from time to time?

put the x axis carriage together ie the rods with extruder carriage getting both rods same length between ends and place it where it should go now take the 2 smooth rods and slide from the top frame down thru x rod ends into z stepper holes you now have the x carriage moving up and down these rods. Next you slide one screw from top and let it thread down to the coupler repeat with other thread. once both screws are in ( may be easiest if you support the x carriage on both ends with anything) you first push the screw rod onto the coupler as far as it will go down then place the coupler with screw onto stepper shaft aligning with flat and tighten all set screws repeat with other side. You now have an x carriage that doesn't fall down or move up by hand remove any supports if used. But one side will be higher than the other. Now we have several option lay a ruler across both y axis rods,or simply measure from each z stepper mounts to the brass screw nut on each side You now turn the coupler/stepper/screw assy until both ends of the x axis assy are same height from top of each z axis stepper base best to measure side with x stepper and simply adjust opposite side. if you used a level across both y rods and they are level you can double check x with level otherwise use tape or vernier to get both brass lead screw nuts same height. Once you get done and can power up and move raise and lower Z axis and recheck that distance is the same it may need fine tuning but if steppers are not disabled and have power when you turn the screw it will require some force and feel like a gear stripping(normal) just overcoming magnetic fields. Check this distance each time you manually level the bed. Check out Electronics Geek website as the have a video on it.