Yea, my silicon is a bit dated: 2x 10core Ivy Bridge xeon @2.6ghz, 1x 12core haswell xeon @2.5ghz.
My choices for B1 across the board are based on the lowest B1 that get k=2 and are bigger than the standard B1 for a Tlevel:
T45 2e7
T50 6e7
T55 15e7
T60 42e7 or 65e7 (65e7 is k=3, which means B2 jumps by 50% over B1=420M)
Each of these is at least as fast to complete the related Tlevel as standard B1 choices, with the bonus of contributing a bit more chance to find a larger factor.
Above T60, memory restricts our choices a bit ekeing out 5% more speed becomes secondary.
