OP, your posts aren't very clear - you can get high off of 20mgs hydro, but take in excess of 120mgs oxy a day? Either way, methadone doesn't really make anyone happy, unless, like me, you take certain risks and use it to hold off cravings for dope and nod to the point of a virtual coma. Whereas higher doses of meth block other opiates, low doses potentiate the fuck out of them - taking 10mgs of methadone after smoking a bag of no.3 dope and taking half my usual valium dose knocked me out hard for over 12 hours not long ago, and I can get through 250mgs ox oxy in a day (not dick-sizing, I'm just a super-sized fuck-up when it comes to such things). I wouldn't take any more. Like the poster above said, save it for a taper. If you are hooked, that is, it's a little hard to tell.
Meth really isn't like other opiods, even subs - long onset, long duration, and a 'high' which many opiate users don't consider a high at all, just a barrier against w/ds. Personally, I use it to prevent myself from wasting money on poor dope and because I enjoy the heavy nod that can result from certain combinations, but that's unsafe behaviour reflecting deep depression, and I'm not exatly proud of myself for it. Redosing's generally a bad idea, and meth doesn't mix safely with anything (except weed, I guess).