Making sure that the carriage rails are parallel to the bed is essential to good printing. Making sure that the Z blocks are properly aligned is the imperative before leveling the bed. I have always used calipers, but found it somewhat awkward to make the measurements accurately. I designed the bridge to span the horizontal rails and hold the calipers in a fixed position. I have dedicated a 100mm carbon fiber caliper (<$3.50 from AliExpress) to this alignment gauge.

Basically to use the gauge, I set it to MM (highest resolution) and mount it on the rails next to the left hand Z block and measure down to the Z stepper motor mount. I then zero the caliper and move it to the right side and repeat the measurement. The reading then tells me how much I need to manually step the right side motor to level the rails to the printer. If the reading is negative, the right side needs to be lowered and raised if positive. I repeat the procedure several times until I get a right side reading of zero.

Once the rails are horizontal, the gauge can be used to level the bed. Just calibrate one corner using the paper shim method and then use the gauge to measure down to that corner of the table and zero. By either moving the gauge on the rails or the table fore and aft, you can measure the relative height of all four corners. If the reading at the next corner is positive, the adjustment screw needs to be turned clockwise and counter clockwise if the reading is negative. Repeating the whole procedure should lead to further refinements. This should get the table level within +/- .05mm (.002”)

Note that the calipers I used have 3 buttons below the display, but the other common model with the MM / INCH button above the display will work as well.