It’s not that easy. Impairment, in the case of marijuana, is highly subjective. Those who use a lot soon lose much of the impairment of infrequent users. Likewise, body weight and food consumption, and just individual differences make a single standard difficult or impossible.
Finally retention in the blood is so great that someone who would have been “baked” when they had freshly consumed marijuana and for a few hours thereafter, will have close to those same levels, but be entirely clear headed, days later.
http://en.wikipedia.org/wiki/Thc
To make things even more confusing, there are dozens of still therapeutic, but not intoxicating, chemicals in marijuana as well. If someone is using marijuana for one or more of those, not for its THC, the type of marijuana used and the dosage will be considerably different.
For example, only one variety of marijuana seems to be very efficacious in reducing the damage from the blindness causing disease Retinitis Pigmentosa. So its users are far less interested in the THC level than in the other chemical responsible for this effect.
Yes, this will be very difficult. Right now in CA (prolly many other states), you can take a couple prescription vicodin at 7 pm, get stopped the next day and fail the drug blood test and be charged with DUI. MJ will be very hard to quantify.