[Sergei]

I checked the results of all 3 calibrations by rotating the TCP around a tip in A, B and C and none was very good, I guess around +-1 mm.

Are you making rotations using a teach pendant?

Try please to write a simple program with OAT rotations around the tip ( try to hold the deadman switch all the time while you record points to avoid 'backrushes' when you release the trigger to record points). Then you can check real deviations from the tip running this program.

In my practice, I'm mostly using tip-to-tip methods.

Will TwinTool gererate more reproducible/accurate results? Is it possible to get a trial license for a week?

Are you have a linear计传感器?

You can find more info below the link:
//www.sinclairbody.com/doc/en/Robot-Automatic-Calibration-TwinTool.html