Re Lines

I’m trying to work out how to set a default Re Line that is ‘our client’s lastname’ & ‘other party’s last name’. I know how to get into the smokeball settings and set an auto re line but the only options are for their fullnames. We really don’t want that. We, and all other firms in our area, only use last names. Has anyone had any success with this? I don’t want to have to manually enter in a Re Line on every matter and then every time we open a file.