How much data do you expect to be going through in this report? I'm concerned because using a Crystal formula for this type of filter will probably cause Crystal to pull ALL of the data into memory and then apply the filter instead of pushing the filter to the database for processing. This means that lots of data will be travelling over the network and into the memory of the computer where the report is running where Crystal will process it to potentially return just a few rows. So, this will slow down your report.
However, there are a couple of ways around this. What type of database are you connecting to? How are you getting the data - are you linking tables in Crystal, using a universe, using a command or a stored procedure?
-Dell