There most definitely is not a safe way to inject fentanyl
patches, this stuff is EXTREMELY dangerous for every way to take it apart from as directed. There are a few threads on fentanyl patches like the thread How to Abuse Fentanyl Patches
which does have some very bad advice in it as well as good, so best to read the whole thread.
There is a PDF i read called Fatal Intravenous Fentanyl Abuse
, this shows how dangerous these things are.
I think these things are way to dangerous to play with, judging doses is extremely difficult. Even if used as directed for the full 3 days there is still a potentially lethal dose left in the patch.