Working in an imperfect manufacturing world in which one surface finish parameter is not directly convertible to another is like trying to make an apple pie with oranges.
Here's a question that is all too familiar to surface finish measuring equipment manufacturers these days:
"We received an order from a foreign manufacturer, and we are not familiar with the surface finish requirements on the print. They specify Rz and Rt parameters, but we use only Ra because our gage only measures Ra. Is there a correlation between Rz and Ra, and, if so, what conversion factor should be used?