Stephan the only person who can accurately answer that question is you by trial. While some general comments can be made about it the tune of your car, traffic conditions, and the setting of your low fuel indicator are all variables. If you really want to know take a can of fuel and run it dry. Then repeat the experiment at various speeds.
Your lamp flashing on and off was caused by the fuel level changing due to shifting of the fuel. Once it was on you were pretty low. If you read your handbook you will see that no absolute data is offered as it is impossible.
You chose wisely!