"Revising" Photographs: When Is it Fair Use?

Dear Rich: I am writing in concern of my use of already existing photographs as a source for my artwork. As a print maker, at times, I base my work on the photographs of others. Sometimes I am able to gain permission from the creator of the photograph and other times I have no luck with the correspondence at all. For the work itself I build a theme around a source photograph. From that photograph I cut a stencil that re-creates the image in a graphic way. The finished product may be recognizable to it's original source but is not a direct replication. I make a very limited run of my final work ranging from 1-23 pieces. Some of the work is produced for gallery shows, murals, or to be sold as prints. Sometimes the work finds it's way into a book documenting art. So my question is where is the line drawn between fair use/transformative use and copyright infringement for images? Asking the Dear Rich Staff to define the line between fair use and infringement is a little like us asking us to define the line between art and nature. We can define fair use (and we've done so at this site) and you can review caselaw and look for comparative fact patterns. But in the end, to paraphrase Marshall McLuhan, fair use is anything you can get away with (P.S. we love MM. Can you believe he wrote this 45 years ago?). Fighting over a fair use claim comes down to a lot of time spent with lawyers and -- as in the recent Richard Prince case --  the outcomes are not always favorable to artists.