General rule is to notice, that specifications of grain is not a reference dimension to RA values.
The specification of grains for a stainless steel surface,means the last touch on the surface of the grain label for shapes etc.

there is no measurable value or conversion factor to specify the grain on a grinded surface !!
Cutting speed connected with various parameters such as diameter from the contactwheeel, length of the abrasive belt,wet or dry sanding and a lot of more reasons finally induce the recognizeable  surface.
Coarse and fine each lies in the perception of personal viewing and the own perception