CSS3 LAYOUT CHOPS - Practical CSS3: Develop and Design - page 265

Let's briefly explore the other two arguments and their effects. Try setting
these values on your properties:
#cast {
flex: 1 400px;
}
#facts {
flex: 1 250px;
}
#plot {
flex: 1 250px;
}
In this case, you are setting widths of 400, 250, and 250 pixels on the flexible
containers; the remaining width is then divided equally between them, because
the positive flex is set to 1 for all of them.
Now try these values:
#cast {
flex: 1 400px;
}
#facts {
flex: 1 400px;
}
#plot {
flex: 1 500px;
}