I think the only way to do it properly is by %. So for example you'd have first crack and second crack so a medium roast would be between 25% and 75% past second crack, then medium dark would be 75% post first crack to the start of second crack, etc.
Terms like city, city+, etc are much more descriptive than medium, dark, etc. For example some people call the start of second crack medium whereas to me I call that dark.
And don't even get me started on what an "espresso" roast is!