Question about FindAGrave research and entries.

I thought there was a post about this lately but cannot find it. It is probably Legacy Searching 101, but I cannot figure it out. How can I tag those who do NOT have a FindAGrave citation in my file? I want a list so I can go and search for those people that are missing a FindAGrave source.  I tried to do a detailed search, but the only choice under either Source-citation or Source-Master Source is "Contains" (under How to Look).  It needs "Does Not Contains"!  I cannot use the Missing Sources tab under Search for Burial Date and Place, because that might not include all who should come up in the missing FindAGrave search (as someone might have a source there, but not a FindAGrave one).  Sorry to ask such a basic question.  I'm sure someone has figured this out before. This information would actually be helpful to know for searching for people with any source that does not exist for a person.