I think any amount of alcohol in your body must affect your driving to some extent, trying to work out the limit that is still safe seems wrong to me - just don't drink at all if you're driving.
My husband thinks if he doesn't have much and it's with a meal then it's ok, it's one of the few "safety" issues we disagree about. I hate it when he drives after a couple of drinks, no matter what he says about safe levels and food etc.