I would build a special calc.
And set your date rage via a start-date parameter and an end date parameter.
Your denominator calc would say
if [arrival date] >= dateadd('month',-1,[start-date parameter] and
[arrival date] <= dateadd('month',-1,[end date parameter] then [FTE Count] END
You'll have a calc that will have nulls in all rows except those where the proper month-ago range is met.
SUM([Denominator calc]) will be your proper denominator.
Caveat: If you filter dates with a quick filter, and the user selects a range that does not include a month ago, your SUM([denominator calc]) will be zero.
Another way to address this -- and allow filtering to work -- is to make a duplicate of your data source. In the (copy) data source create a calc like this:
And then blend your original source with your copy source, and join [Arrival Date] from the primary and [Arrival Date Join calc] to make the connection. All the data from the secondary source will be offset by a month, and if you just grab SUM([copy source].[FTE Count]) from the secondary source, you'll get the previous month's data connected to the current month's [Net Revenue] from the primary. In this way you probably wouldn't even need the start-date calc and end-date calc.