Does this help:- http://www.greenbank.org/sob/p-1.html

http://www.greenbank.org/sob/p-1_limit.html is for 9M <= n <= 11M

The tables show the percentages (in brackets) of factors found that would have been found with those B1 and B2 values.

Haven't updated it for a while but it should be a good place to start.