2 things that'll make any grading system virtually useless:
The weather: Today's easy dry dirt road is tomorrow's muddy quagmire after a night's rain.
The rider: What's easy to a competent rider on a well set up, appropriate bike, might be a nightmare for someone with poor ability/ fitness on an unsuitable vehicle (think born again, all the gear, no idea on a faux 250kg+ heavy bomber with lots of plastic and twiddly bits).
 Sorry (again  ) for all the negativity
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