Hi Mick, welcome to the board. Under the circumstances (chronic pain which is never a treat for anyone) you have come to the best place I've found o line for support. I don't have much advice on your question, but I'm sure someone will be along later tonight or tomorrow (when everyone wakes up) that can be of better assistance. I sis Google search and I found a calculator but I'm not exactly sure I know what I'm doing so I'll post the webpage here:
http://agencymeddirectors.wa.gov
I hope you get the answers you're looking for but I will say, it seems like going from Tramadol to Fentanyl is quite a jump! I hope you find relief soon. Welcome again!
Linds