Thanks George. I was referring to the skipping. It is interesting to see that k affects the optimal bounds so much and makes P-1 "seem" even more non-deterministic than in GIMPS.

I've read the source code that chooses P-1 bounds several times. I believe I know how it works. And it is not intuitive. So near FFT boundaries one cannot really say what bounds will be chosen. But I am convinced that it is optimal.

Finally, I believe that 48 is the correct bound for the moment (i.e. for 6M exponents) as only 10% of the range above 2^48 has been sieved. So we are really at 48.1 and not 48.55.