Weavus:
I grade the coin for the details first, giving me a numerical grade. Then I look at the "issues" a coin might have such as bag marks, nicks, scratches etc. These are not in the initial detail grade but will bring the value of the coin down from the detail value. Those individuals who collect Early American Coppers will actually revise the original detail grade to reflect the "issues" and come to a net grade. Then there is the overall appeal of the coin, is it lousy, average or choice for the issue.
Deductions just like the original grade are subjective.
Hope this helps.
KK