Calculate a ranking function from classification features
Generally speaking moving from a classification space to a ranking space is not straight forward. In classification problems there is no meaning to order between labels. This means that your suggested equation might not represent the order between labels at all.  

This is somewhat dependent on the feature space and classification algorithm. Some classification algorithms(tree-based for example) don't use the concept of distance in their search for best fit. The separation created from the fit(which is used for classification) does not include how far the boundaries are from one another.

Some classification algorithms (logistic regressions, SVM, others) do have this distance feature, and this can be translated to the probability of an instance belonging to each class. Using this probability as a ranking mechanism (instead of how asking how efficient, asking what is the probability of being efficient) might make sense.