Grading is tough because although it seems that it would be objective, there is a strong element of subjectivity too. For example, given two guns that have exactly the same amount of wear and handling marks so that objectively they would grade out identically to 95%. Now let's say that one has had less than optimum storage and care so that it appears dull (not pitted or what the dealers call "frosted"), but still 95% original blue, and the other has been well cared for (perhaps a Swiss gun
) so that the bluing appears bright almost as new, you can bet that the first one will be graded out at 90-93% and the other probably at 97% or better. And to be practical, the better looking one is really worth more even though they both are "the same" percentage wise.
Is it necessary to grade both on coverage and condition? Perhaps a percentage rating accompanied by the modifiers "dull", "average" or "bright"? Maybe that is what the sellers are trying to convey when they advertise a piece as 97% ++ .
A good grading standard would be a real boon, but as I said at the beginning, it is tough.