The legal blood alcohol limit is .08%. That is very low and equals about one drink per hour, but that is very subjective. How you process alcohol greatly depends on your body and a variety of other factors. For you, one drink per hour could be too much to legally drive.
Remedy Health Media explains it is difficult to know your blood alcohol level without testing it. You could easily be over the limit with one drink, so unless you have some way to test your BAC, you should probably not drive after having a drink.
No way to know
The .08% limit is so low that you could be too drunk to drive without feeling any effects from the alcohol. You also have no idea how your body processes alcohol, and unless you drink the exact same product every time, meaning it is not a mixed drink but comes premade with a specific and consistent alcohol content, you cannot even gauge an estimate of your BAC based on what you are drinking.
Furthermore, since the factors that determine BAC vary greatly, you may have something different about you one day that causes the alcohol to not process quickly enough through your system. The factors could change consistently, so you never can be positive about what is going on inside your body.
Zero is safe
It is tricky and not safe to try to guess your BAC prior to driving. Since there is no real way to know your blood alcohol level, having nothing to drink if you plan to drive is your safest option.