There was a DEA report some time ago that pegged 95% of 'street tabs' as being dosed between something like 35 and 85 micrograms, when they contained genuine LSD. This stat has probably changed some in recent years in a positive trend, but I still don't bother with street tabs personally. Almost every street dealer I've spoken to has advertised their tabs as 2-300+ mics per tab, called it double or triple dipped, or other BS marketing tactics.
As Hodor said you have much better chances on the deep web for getting what you pay for, particularly when dealing with smaller quantities like a few tabs. Because they are slightly more accountable, other people will leave bad reviews if the guy's stuff is bunk.
In terms of what is the average dosage offered on the deepweb, I would say 150-180 micrograms seems to be the more common dosage advertised. I think it must be easier to get more money for stronger tabs online vs on the street, where 1 tab = 1 tab, so that's another factor in favour of the deep web.