The dosing for oxymorphone is incredibly complex. Comparing clinical oral dosages have basically nothing to do with the power of snorting or IVing it due to massively different BA's of each ROA. Best bet is to snort micro lines until you feel like you're at the right spot.
Edit - the guy above me is giving dangerous advice, the drug is far more powerful when snorted. If oxymorphone is twice as strong as oxycodone when eaten but only absorbs at a 5% BA, then sniffing it is around 50% for example, then where eating 10mg could feel like 20mg oxycodone, sniffing it is 10 times stronger so using it that way you'll see that about 2mg snorted will feel like 20mg oxycodone. Roughly.