Cun is the traditional body-relative unit used to locate points, about the width of the thumb at the knuckle.
In 📝Acupressure PC6 is "three cun above the inner wrist crease" — three of that person's thumb-widths, not a ruler. The unit is a locating tool, not a diagnosis, and not a claim of precision as a medical measurement. Landmarks plus feel still decide the point.
Clients do not need the word. Homework uses finger-widths of their own hand.