This doesn't directly relate to the OPs question, but I thought guys might find this interesting:
Here in New Zealand meth has become the most common/popular 'hard' drug out there...over the last 10 years or so NZ has gone from producing pretty much all meth used in NZ, to there being a real mix of NZ-produced as well as a lot of Asian produced shit (Burma, Indonesia & Thailand apparently? No way of knowing really) to the current situation where around 75-90% of meth on the market at any single point in time will be imported from Asia.
I'm not a big fan of meth- or stimulants in general- so my perspective is that of a predominantly non-stimulant user.
The DIY, made in some Maori/skinheads garage/barbeque/wool shed. The quality, colour and general level of impurities varied heavily, although I'd hazard to say that most of it was from poor production and a lack of any/proper purification...but if it was cut it was generally only lightly cut, friends of mine wash their meth regularly for IV (easier to purify 3.5g than micron filter 20-30 shots) and it's almost always ~75+. Making good quality meth isn't hard- most of it here made commercially uses ephedrine as its feedstock, it's really not hard. It's just whether or not the cook can be fucked and if he's willing to sacrifice the extra earnings.
Different colours would often indicate different qualities- I don't know them all, but I can remember yellow meth meant that they used excess iodine to ensure a full conversion of the ephedrine into methamp- so yellow shit tended to be strong, if stinky and obviously impure. Red was terrible to IV, what with all the red phosphorous sitting in it. Different candy colours tended to mean that the dipshit managed to turn pseudo into methmp, but didn't think to clean off the coating- meth does not necessarily make you as smart as you feel. Shake-n-bake is starting to gain some popularity, but it's almost always made for personal use as it's garbage and because...
Asian methamphetamine is the bomb. Whatever you get will most likely be brilliant white, cloudy clear or at worst a very light-piss-yellow (which isn't is a bad thing^). As I said cutting isn't that much of a thing here in NZ- misrepresentation and mixtures of drugs being sold as other things or under made-up names is rife, but cutting is rare- so the quality tends to be very consistently high. People have purified it- again, for IV, as well as curiosity- and it's almost always been 85%++.
As Ebola said- methamphetamine is just a chemical, you don't even want 'all' of it, just the dextro-isomer thank you very much. Breaking bad was quite realistic to a point, and it's still fairly well researched intelligent show, but all the stuff about cooking meth being difficult or chemistry requiring some sort of genius level understanding of chemistry is bullshit. The only reason you might need a university education to work in a superlab is to learn how to operate the various machines- but even then a lot of industrial chemists would be more familiar with such industrial scale equipment then even someone like Alexander Shulgin (who could do it- but he'd have to read the manual or be given a run trough). But really, file under poetic license.
As long as the cook goes to the trouble/time/cost of purifying their gear and IF it manages to work its way through the supply chain without getting stepped on too much then 95% pure meth made in a trailer should be indistinguishable from racemic meth made by GSK or Roche or Pfizer or whatever.
I would speculate though that the big meth producing cartels (for instance, in Burma, Indonesia, Thailand, Mexico) can afford to absorb the lost profit and consequently I'd imagine that their cooks would go to the trouble to producing virtually pharmaceutical grade meth...which is not to say that it's not cut further down the line, even within their own organisation. However by starting off at ~99% pure rather than ~72% pure when your ass pirate of a dealer decides to cut it heavily to make "FAT STACKZZ YO!!elevenone!" the cartel gear will still be ~49% while the DIY gear will be a pitiful 22%.
Plus with cartel gear the gram of meth that you have in your hand will be out of a kg bag, which is one bag from a production run of 250kg in the jungles of Burma...there's probably a lot of meth exactly like yours sitting around your city, so as long as the cut doesn't change too much there will probably be a level of consistency you just can't get from dealers who depend on cooks who only does 1/4lb runs as often as he can smurf the shit he needs. And shake'n'bake would be even worse I imagine- tiny runs of horrifically impure meth.
Jesus that was rambly, I'm not even on stims- I'm on smack!