I would pick something like 50, because my opinion is that people are impaired at a concentration of 50 milligrams of alcohol in 100 millilitres of blood. Just for your example, you could have somebody who is not used to drinking who could be impaired below 50, and then you could have somebody who is used to drinking, who routinely consumes alcohol, and they might not demonstrate outward signs of intoxication from the alcohol consumption. They may be able to get from point A to point B without too much difficulty. If they are challenged on the roadside, meaning if there is a sudden or unexpected task, that's when impairment becomes an issue.
The same goes for THC, in that if you are a chronic user, you do develop some tolerance to the drug. That doesn't mean the drug is having no effect on you, but it does mean that you would have to increase your dose to achieve a similar effect to what you are looking for, so there can be people who are impaired at quite a bit lower level than others.