You use it to convert your fentanyl dosage to an equivalent morphine
dose, and then find the equivalent morphine
dose to oxymorphone,
Generally when converting between opiates, doctors will reduce the new medication strength by a certain percentage to account for incomplete cross tolerance, which is a boost in analgesia you get from rotating from one kind of opiate to another.
You can find equal analgesic tables, and apps for calculating equivalent dosing online.