Another way I could do the formula would be to use:
RUNNING_SUM( [Policy Number Count (Distinct)] ) * (12 / MONTH( [Termination Date] ) )
but the running_sum function isn't working for that either.
The following article should give some insight on resolving this issue:
The calculation should be edited to look similar to the following:
((RUNNING_SUM( [Policy Number Count (Distinct)] ) / [Total Count] ) * (12/MONTH( max([Termination Date] )) )
Hope this helps!