As a few people have mentioned already, there really is not a foolproof correlation between "amount taken" and "degree of impairment" for any drug. Even for alc

As a few people have mentioned already, there really is not a foolproof correlation between "amount taken" and "degree of impairment" for any drug. Even for alcohol, frequent alcohol users can build up significant tolerance. In general, blood concentrations for THC are pretty good at identifying impairment for infrequent users (greater than 90% predictive in some studies), but are very inaccurate for frequent users with a built up tolerance.