If ( PatternCount ( Customers::Address 1 ; "Maxwells Green" ) ; Do what you want when customer is from Maxwells Green here ; Do what you want when they aren't here )
PatternCount and If are functions that you can look up in FileMaker Help. Case could be used in place of IF here.
But you may want to rethink that a bit. At some point in the future, you may want to change the "rules" for who get's this discount. Such a change will require redefining this calculation each time. There are ways to set up matching to a table of values so that you can edit the records in this table to control who get's a discount so that updates become a data entry task instead of a database developer job.
Yes, down the road (in the next month or so) I am going to create a discount table for that reason, but for now I just need to get this database up and running and that's the only discount being offered, so it'll do. Thanks for the input and help, I definitely appreciate it!