D,
I did find a few grit conversion charts....
http://www.ameritech.net/users/knives/grits.htm
http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.html
http://www.reade.com/Sieve/grit_conversion.html
http://www.faceters.com/askjeff/answer67.shtml
http://www.unc.edu/~rowlett/units/scales/grit.html
http://www.facetingmachines.com/grit-mesh-micron.shtml
...that maybe helpfull in determining the size of grit.
Check them out,
John Wright