[deeppow]

I'll depart from my normally hidden mode.

Surface roughness is a fractal characteristic --- what the heck does that mean?! It means the measurement is dependent on the tool (typically stylus size) used to measure the average.

The specification should state the measurement and the tool used in the measurement of the average quoted. Otherwise the average and variance are not unique. The requestor has given you a specification that will depend on what measurement tool you use and there a several answers.