the only real way to know is to send your blotter off to a lab to get it tested. which i've never done, because blotter is f'n hard to come by where i am so when i'm lucky enough to find a couple hits i don't want to mail them away. also everyone seems to have a different tolerance level depending on their experience, body, etc
the first time i dropped, i took one hit and it absolutely blew my mind. time completely stopping, people freezing in their tracks, objects melting..meanwhile everyone else who was more experienced was complaining about how weak the acid was =)
i'm sure there are those on the board who can give you an answer from experience, but generally from what i've read anyway a light dose is around 50 mics pretty much going up from there, a strong dose being ~150mics and up. every blotter i have run into has been very, very different. a half hit of good rolling stones blotter had me tripping hard for 12 hours. two hits of street blotter hardly did anything at all on the other hand