I agree with T. on this one. Methadone is a potent Opioid by any route of administration. Taking your current Oxycodone habit into consideration, I would be willing to bet that 20mg would be at least sufficient to keep you straight. Be aware that it's probably going to take 2-3 hours to experience relief from withdrawal symptoms if consumed orally, but you will begin to experience some relief ~45 minutes to ~1 hour post-administration.