In my opinion, no value can be derived from the single data point of "steepest". What you really care about is "difficulty", and difficulty is some strange combination of grade and length, plus some factor for how steady the hill is (i.e., a hill that is unrelenting is more difficult than one where the road levels out from time to time to give you a break). If the hill is only 50 yards long, the grade is almost meaningless (unless it's so steep you cannot even pedal at all, in which case you can just walk it, unless you cannot walk, in which case you really do care about grade). Of course, a couple hundred 50-yard hills in a row can wear on you.
As far as I know, no one has successfully come up with a agreed-upon numerical way to express difficulty.