Yeah I don't pretend to know purity. My personal (tolerance dependent, of course, I have a moderate-heavy tolerance) rating system is pretty much as follows:
Crap = Usually minuscule bags, takes about a bundle for an OK shot, pray for a 20 minute nod! Noobs can still get kind of high off one bag.
OK = A vague category. A lot of times bags will fall into this category simply because they are too small, even though the dope is good quality. Anyhow, anywhere from 5-8 bags for a 'good' shot, possible nod. Noobs are High off one bag.
Good = Usually good size, sometimes stinking of acetic, 3-5 bags is a good shot. Nod is likely. Noobs are TOAST off one bag, sometimes off half.
BOMB = God I sure could use a batch of this stuff for the holidays, but that's most likely wishful thinking. Pretty much always damn stinky, either of acetic or thinner smell (usually thinner/chemical if it's the real white stuff), IMPORTANTLY this shit will get me PRETTY DAMN HIGH off 3-5 bags SNIFFED. The last shit I had that was 'bomb' gave me A RUSH when insufflated. Deny all you want people, I'm experienced and I know what I felt, and it sure as fuck wasn't placebo as I didn't have high hopes for the dope in the first place. I mean fuck, I didn't even want to say anything positive because I was so amazed until my buddy looked at me and was like WTF I'm WASTED!! My best guess about the rush was that it was caused by some very harsh chemical that was in it, the shit burned like hell nasally, and tore up my nose a lot, I guess maybe because it was so corrosive it expedited the absorption nasally??.
ANYWAY..... Yeah bomb shit is 2-3 (3 MAXIMUM) bag AWESOME shot. Nod is pretty much guaranteed, and heavy. Not recommended for noobs, one bag can put a noob in the hospital. My friend idiotically chose not to believe us and shot 2 bags at once (he has a slight tolerance) and we ended up tossing him in front of the ER.
Wow I really went off on the bomb shit huh? Guess it shows how passionate I am about it. Anyway, I know some of you down in NYC/NJ CAN get better shit, shit that would probably knock me flat with 1 bag, but the scene where I am is removed from the main points, thus the dope tends to be lower quality. At least I only pay 40-50$ a bundle, MAXIMUM 60-65 for the BEST shit, if my guy is not around to sell it cheaper.
Oh and I have a thought about the whole DEA reporting qualities thing. I am pretty sure that a LOT of their testing is done on large(er), supply side batches of seized dope. Of course if they test a straight kilo, it will be higher % than the bags that are then made out of it. I know they have tested individual stamps before and shit, I just think that overall that is why their numbers are so damn high! I mean the average user simply CANNOT EXPECT to get street dope over 50%. Of course it happens, but if you were to actually test every batch on the street level, the numbers would be much, much lower.