Slashzor is correct.
What he left out is that most have discovered if you have drivers change at 10% of strength level, it doesn't give the other drivers time to recuperate. Thus once through the rotation of drivers, they tire frequently and therefore pit more frequently than necessary.
I used to use the 10% strategy, but now leave it at default. My drivers do just fine at 0% mental and physical and come in for a pee stop when the car needs tires or fuel.
Your milage may vary,
Kaffeine