Exclude specified filter

ksimpkinson , As per what I got it

Turnover Rate =
DIVIDE( [Total Terminations],
CALCULATE( [Average Employee Headcount], ALL(Table[Detailed Action reason]) )

or
Turnover Rate =
DIVIDE( [Total Terminations],
CALCULATE( [Average Employee Headcount], removefilters(Table[Detailed Action reason]) )

Sometimes this does not work out at that time you need to a separate table for Detailed Action reason and you use all on that

Turnover Rate =
DIVIDE( [Total Terminations],
CALCULATE( [Average Employee Headcount], removefilters('Detailed Action reason') )

ksimpkinson - You can use ALLEXCEPT or REMOVEFILTERS or KEEPFILTERS to edit context of the calculation.