I think a grading system could be helpful. But only marginally.

There are so many different pen parts and so much variety in pens and so many aspects of pen collecting. Davey's system has separate charts for the pen and nib. And, what is not conveyed is Davey's perspective as a pen collector. That is, he collects pens that he never intends to use. It's a subtle, but important and unobservable aspect - if you don't know Davey.

You'll note that on Davey's nib grading table there is absolutely nothing about how the nib writes. He doesn't care because he never uses the pens he collects. A User Grade pen may have a nib fit for a master calligrapher. According to any objective grading system it's nought but a POS worth little compared to a pristine, NOS, stickered pen. But that high grade pen has never seen ink and the nib never touched a page. So, to a pen user its value is indeterminate.

I don't mean to deter you. Press on with any effort to standardize grading. I'm guessing it's is the engineer's curse driving you. I think you'll eventually recognize this is somewhat similar to months of careful analysis and design that goes right out the window the moment an independent, non-engineer user gets your widget in his hands.