1. A method for determining a subset of a set of library cells of a library cell type for a timing arc of the library cell type, the method comprising: receiving a first generic logical effort value for the timing arc of the library cell type; and determining, by using a computer, the subset of the set of library cells of the library cell type by excluding library cells in the set of library cells of the library cell type whose specific logical effort values for the timing arc are greater than the first generic logical effort value by more than a first threshold.