Jekis, if you use Menu option Setup -> Grading, you can see the relationship of the various Grades as offsets from NM. For example, for the base Grades:
Mint is 105% of NM (all of the Percentages are of NM value) NM is 100% (naturally) VF-NM is 80% GD & FN is 33 % VG is 20% G is 13% FA is 8 % P is 2 %
For CGC Grades
CGC 10.0 is 2800% CGC 9.9 is 2600% CGC 9.0 is 1200%
and so on.
A CGC 8.5 is 110% while a CGC 8.4 is 85% so NM would be between those two (closer to CGC 8.5)
Actually Grades like 9.4 or 9.2 (outside of CGC or PGX) are not listed.
Not sure if that helps or not. It would depend what number you would apply to Mint (10?), NM (9?), and VF-NM (9.5?, 8?). Whatever the numbers would be, you can use the Percent of NM listed to get some idea of the offset percentage.