You can still be considered legally impaired
below the
presumptive limit with alcohol. Depends on the person. It's not like the presumptive level is forever chiseled in stone, and it's been lowered over time (FWIW, it was 1.0 here in CA many years ago, and then was lowered to .08).
The THC impairment is a bit thornier in that respect, as it's the objective signs of
impairment due to THC that have to be used. Last I heard, it's not presently considered possible to develop the same sort of presumptive chemical test to show real-time blood levels present in the way done for alcohol testing.
Here's a lay person explanation in the media:
State House leaders expect to soon help define what constitutes driving under the influence of marijuana. But deciding what to require may be difficult.
www.wbur.org