This is the first tweak listed in the parent ticket and includes the following changes:

1. (flip default to false) don't ignore mortality rates of cohorts older than previous tenuring threshold plus 1.

2. When the newly calculated tenuring threshold based solely on mortality rate as computed by the current algorithm is less than the tenuring threshold used in the previous epoch, then "xeno down" to the target new threshold by going halfway down (rounded down to an integer value). This makes the lowering of the tenuring threshold proceed more slowly than was previously the case.

Performance numbers will be included below.