We have shown in previous studies that the entanglement molecular weight for a polymer melt, M-e, is related by a power law to p, the packing length of the polymer species. We now find that power laws also describe the molecular weights characterizing the melt viscosity, M-c marking the onset of entanglement effects and M-r the crossover to the reptation form. The packing length exponents for M-e, M-c, and M-r differ significantly, however. The long-held notion that the ratio M-c/M-e has the same value for all species is therefore incorrect. Further, the observed and predicted values of M-r for two species, 1,4-polybutadiene and polyisobutylene, have been found to agree, within the uncertainties, with the projected values. Finally, the variations with packing length are such that all three characteristic molecular weights would appear to converge on the same value near p = 9 Angstrom. As yet, no species with such a large packing length has been completely studied theologically. But the range is not outlandish and is clearly reachable by appropriate synthetic methods.