argh, there's good reason they've mainly gone polymer matrix with fent patches, it's that damn potent and the gel reservoirs don't really allow for proper guidance with dosing. essentially it's start with a tiny dob the size of a match head and put it under your tongue/cheek, wait 5-10mins gauge how you feel and SLOWLY titrate from there.
i'd try and trade it for something else really because of the OD risk associated with fent and because it pales in comparison to most other opioids.
please be careful.