The reason methadone is so deadly compared to other opioids is its half life (and accumulation - this is a key word with methadone), nevermind its high potency.
For example, a totally opiate-naive individual gets his hands on 30 mg of methadone. He takes 10 mg, waits a few hours nothing but a little sedation. So he takes another 10 mg, an hour or 2 go by and nothing but sedation. He doesn't notice he's nodding. So he takes the other 10 mg. Two hours later he's dead. That's a thing called accumulation with methadone. So you always have to be wary of the fact that methadone accumulates in your system.
So always start low, I'm not going to tell you what dose to start off with because only you know your true tolerance and how much your body can handle, just like the rest of us. But start low and work up from there. Remember if 10 mg (example dose) don't work, wait until tomorrow and take maybe 15 mg. Do not redose with methadone. Like I said, for example if 10 mg don't work today, wait until tomorrow (24 hours) to try 15 mg (example dose).
Always remember that you can take more, but you can never take less.
Have fun, but be safe!
