Thanks Mikael, I read that it takes a 30 fold increase in processing power to increase difficulty by 1 . . . if so difficult to imagine difficultly ever climbing beyond 12 or 13.
999/(13*13) = 5.9 per block
i thing there are about 1000x optimization left in the code....at least
so . . . 999/(15*15) = 4.44 per block