Great work, Matt!
I disagree with regards to binning the grading system, Simes, because some buyers are not like us.
The grading system helps a new buyer and helps when they come to sell an item.
I created this comparison years ago from a lot of research and discussion with poster, art, toy and memorabilia collectors and sellers (including auctioneers), and use it on my websites today in the help sections:
I currently use the 6-Point Grading System myself - obviously "Mint (M)" and "Near Mint (NM)" are replaced by "Very Fine (VF)" and "Fine (F)" for paper-related items because technically-speaking a piece of paper cannot be classed as "mint" or "near mint". I also spent a lot of time on Jon Warren's guide, even speaking to him about it, and making it simpler to read and work from.
However, Matt, what you have done with it is brilliant
It's making me rethink my decision to completely ditch it!
The biggest problem the C10 system has though is with the public. For them, and I don't mean us nutters here, it is inherently complicated, which is why many businesses use a 5 or 6-point method. I'm an advocate of the C10 way, but I just don't think posters will ever have a standardised grading system in the same way as comics because of the retail aspect of posters and prints these days.
For me, and where I want to end up, the C10 method will probably be more of a hindrance as my Gallery will form only part of my business.
I might end up using what you have done Matt as a great tool, along with my own observations, and converting the relevant score to a grade commensurate with the 6-Point system, so thank you for what you have created