When you say "...filters out the last 7 characters...", do you mean you want to keep them or remove them?
If you want to keep just the last seven characters, use this:
Right ( field; 7 )
If you want to chop them off, use this:
Left ( field; Length ( field ) - 7 )
But I would be interested in why you want to keep or chop the last seven characters ;) I suspect it would be better if the calculation was smarter than that. But it may work for you OK.