Hi durkie
The 320 grit reference is really an indication to the size of the grit in an abrasive product and the conversion to surface texture Ra is probably based on the finish obtained on steel using that product (not necessarily same as carbide coating).
I can only guess that someone has compared say the texture of a 320 grit silicon carbide grinding paper with that of a as-sprayed coating in a sight and feel sensory fashion. I would say that they are probably not too far apart
, but texture of abrasive grit (or as-sprayed coating) is far different to the actual finish obtained on steel using that abrasive. Hope that makes sense