Legally it's 80mg of alcohol in 100ml of blood.
But since there is no way of you telling how much alcohol you have in your blood while you're at the bar, or how much alcohol that's going to be in your blood when you turn on the ignition (alcohol does take a bit of time to be absorbed), you're better off erring on the side of caution - don't drink and drive!
Also, how alcohol affects you depends on a number of things - how much/regularly you have been drinking over the past few weeks, how much you've had to eat before hand, how stressed/emotionally affected you are before/after the drink, your bodyweight, etc. You'll only find out that you're beyond tipsy when you get to that point, and when you do it's no use kidding yourself that you're fit to drive. It's outright dangerous and irresponsible.
Be strict to yourself about it. Find some other way to get home, or no drinks at all.