It's simple, the level should be zero. That way all drivers know not to drink if they are driving a car. Very simple and clear for all parties. Simple to enforce too.
The present system is the dumbest possible thing because it tells drivers they may in fact drink alcohol but they must know subjectively when to stop drinking, which gets harder to do once consumption has begun. Then people have two questions to answer, am I over the legal limit or not and will I get caught. If they do get caught, can it be beaten?
The easiest most clear cut and god forbid intelligent thing to do would be to make the limit zero, then everybody knows EXACTLY where they stand before turning the key.
But that would be way to easy and make too much sense for any US Government to employ. They would rather employ stupid mottos like over the "limit" (.08) under arrest which is not even remotely accurate or fair to the driving public as it is not an accurate reflection of the law. Thus we have a stupid subjective law (ie. The number changes over time and will continue to do so in the future) which the government itself does not know the ramifications to properly warn the motoring public.
What a ridiculously silly situation we find ourselves in; well, at least until somebody with a brain figures out this is not an effective way to save human life. It is stupid.