I'm not a big fan of putting everything in the cloud, but one of the advantages of online grading systems is that it is easier to make this kind of adjustment. The workflow goes like this: make a rubric item for a specific kind of mistake (it takes a little experience to know which mistakes are likely one-off and which ones are likely to be repeated by other students), assign X points, and later if you decide there are worse mistakes, adjust the points and that gets applied to everyone.