The standard array 14, 13, 12, 11, 10, 9 "prices out" at 21 points. I use that and 15, 14... that "prices out" at 28 points for more powerful NPCs. BBEGs get 32 points, just like the PCs do. Once I remembered that NPCs get that stat bump at every 4 levels, just like PCs, I made even better...