Well, it is easy, use a light meter !
This not a complicated tool and I will write more tomorrow about the way to use it. But here, I want to write why you should use one.
Take a mesurement witht he light meter : let’s say you get 1/125 for f8.
Knowing that 2 stops above your mesurement everything goes white, you only have to count.
Look at the picture above, it was shot in a tiny room at f8 while the mesurement of the light falling on the wall was f22. More than two stops. So the wall became totally white. Which was no suprise because I had 2 strobes aiming at it.
But also work with a model posing in the shade but with a sunny background. What you need is two stops difference and then nobody will see the cars and what ever was standing behind your model.
More to come