i'll be a sheep and say 10mg (for HR's sake), start with 10mg it will get you a feel for the methadone effect wait 3 hours and if its not strong enough, add 5mg wait another 3 hours if you're still not "where you want to be".. that being said 10mg for someone who weighs 60-kg is probably equivalent to over 15mg for someone who weighs 100+kg imo
IME seeing others taking methadone for the first time, and their reaction to 20mg, and 40mg for some of the more stupid friends who didn't take my advice, which was 20mg max, they survived fine (i think nthey did not take any other cns depressants at the same time) 10mg is playing it very safe.. but it also depends, do you want a light high, or do you want nod....
so i guess tbh, dose 10mg, but there is very little danger in redosing 10mg 3 hours after IF the effect of the initial 10mg is very light (and i repeat this is only appliable if you take NO other CNS depressants)
also people some people metabolise methadone way faster than others.... and some way slower, making it a gamble to redose because you don't feel very high, when the methadone is in fact only half way to its "peak" effect (this goes for people who have not used, or rarely used methadone and don't know exactly how they will react to it...