On an extruder, the rotation_distance is the amount of distance the filament travels for one full rotation of the stepper motor. The best way to get an accurate value for this setting is to use a measure and trim procedure.

First start with an initial guess for the rotation distance. This may be obtained from steps_per_mm or by inspecting the hardware.

Then use the following procedure to "measure and trim":

1. Make sure the extruder has filament in it, the hotend is heated to an appropriate temperature, and the printer is ready to extrude.
2. Use a marker to place a mark on the filament around 70mm from the intake of the extruder body. Then use a digital calipers to measure the actual distance of that mark as precisely as one can. Note this as <initial_mark_distance>.
3. Extrude 50mm of filament with the following command sequence: G91followed by G1 E50 F60. Note 50mm as <requested_extrude_distance>. Wait for the extruder to finish the move (it will take about 50 seconds). It is important to use the slow extrusion rate for this test as a faster rate can cause high pressure in the extruder which will skew the results. (Do not use the "extrude button" on graphical front-ends for this test as they extrude at a fast rate.)
4. Use the digital calipers to measure the new distance between the extruder body and the mark on the filament. Note this as <subsequent_mark_distance>. Then calculate: actual_extrude_distance = <initial_mark_distance> - <subsequent_mark_distance>
5. Calculate rotation_distance as: rotation_distance = <previous_rotation_distance> * <actual_extrude_distance> / <requested_extrude_distance>Round the new rotation_distance to three decimal places.

If the actual_extrude_distance differs from requested_extrude_distance by more than about 2mm then it is a good idea to perform the steps above a second time.

Note: Do not use a "measure and trim" type of method to calibrate x, y, or z type axes. The "measure and trim" method is not accurate enough for those axes and will likely lead to a worse configuration. Instead, if needed, those axes can be determined by measuring the belts, pulleys, and lead screw hardware