Well, technically, BL isn't here to help you get a high, especially off methadone.
However, for HR purposes only, I'd say do not take more than 20mg. Are you used to taking 5mg a day or did you just begin the MMT? If you are used to it, as I said, 20mg should do the trick. If you just began the MMT, don't take more than 10mg.
One more thing, do not make a habit of dosing more than what you're prescribed. It's a vicious circle and it can totally destroy your MMT routine.
Wtf you talking about, "BL isn't here to help you get a high, especially off methadone". It's a harm reduction site, to minimize harm from users taking recreational doses of drugs. There isn't some BS discrimination rule against asking about doses of methadone to get high. Just because you feel it should be only used for MMT doesn't mean it can't be a recreational high to some, or that people shouldn't enjoy methadone high if they find it recreational.
I find it can be very recreational, and a wonderful high. I personally love long lasting opioids because I like not worrying about coming down and just having a relaxing day. I know a lot of people say it isn't very recreational, but it really is a subjective thing. You may find it recreational. Honestly I find it annoying when people automatically assume people aren't going to enjoy certain drugs recreationally. Let people decide for themselves.
If your taking 5mg regularly, I'd try 15-20mg first, then go from there the next time you plan on dosing it recreationally.
Also, I know there is really spread around drug web forums a negative attitude about insufflated methadone, but in reality insufflated methadone is very effective and comes on a lot quicker than oral or rectal methadone. You also get a more stable BA while using insufflated methadone. The one issue is of course the irritation with insufflated methadone so to minimize this I essentially use 10mg roxane tabs to create a methadone soln that I filter, then insufflate .150-.200 mL of methadone in each nostril at a time. This helps minimize irritation caused by snorting methadone, and you get the benefit of a faster come up with just as high of a BA as oral administration, and same duration.
http://paincenter.wustl.edu/c/BasicResearch/documents/ClinPharm2002.pdf here the link to a article going over this topic.
edit; since you've obviously had experience with that dosage, I don't see why 30-40mg would be a problem. if your scripted 20mgs, 30-40mg wont cause any issues. Before I started taking 30mgs or so daily 30-40mgs used to get me very right. I'd be able to nod for a good 7-10hours. I can still feel pretty legit off of 60 or so mg, but I don't like to go higher than that or I'm just going to build up to high of a tolerance with less and less linear increase in effects. I can see going up to 40mgs for a day then back down to 5mgs for a little while would allow you to maintain a decent high from methadone once in a while when you feel a high dose is in order.