If the AD users in each region are in distinct server groups (for example "West Region", "East Region" etc.) you can use the ISMEMBEROF() function in a calculation to create a filter Something like
IF (ISMEMBEROF("West Reqion") THEN "West"
ELSEIF ISMEMBEROF("East Reqion") THEN "East"
Then the filter calculation (True/False) is [Region] = [Group]
The first calculation is unwieldy because the ISMEMBEROF() argument can only be a string constant.