Hey man, all we can give you is a rough estimate, but it's definitely something to start with. What you should do, is take these pharmacokinetics, while also taking into account how you feel. Delusions of sobriety are a fact of life for a lot, or even most drug users. I've seen countless friends tell me they don't feel well and start mixing something up when they can't even keep their head and neck upright or their eyes open. So, think about how you feel, but pay attention to some of the more objective indicators of Opioid intoxication, like pruritus (itching), a more than normal constriction of your pupils even in normal light (miosis) etc.
Methadone is a longer-acting Opioid. This is primarily why it was chosen for maintenance programs back in the 60's. It's fairly cheap to male and carries most people for a full 24 hours or more. There is a wide variability among individuals, however. It's unlike a lot of common Opioids like Morphine in that its pretty much homogenous in effects regardless of the route of administration. This still means a dose can really have a different effect from one individual to another. This is primarily genetic, but other variables like stomach Ph also come into play, with a more alkaline environment generally leading to more absorption.
Are you taking it as part of a program or have you just taken it a couple of times? If you've taken it once, you really don't need to be concerned about the "blocking" effect that it can have. If you wait 24 hours it should mostly be over. Peak blood levels are generally reached around 3 hours post-ingestion and most folks, myself included, would say that this is also around the same time as peak of perceived effects, with the primary, strong effects lasting from 3-8 hours before they begin to gradually dissapate.
Does this help?