For all intents and purposes, absolute minimum and absolute maximum initial torque values are open and locked, respectively. There are those who claim they can feel a difference between 5 and 6 or 59 and 60, but I think they're full of it. From what I've experienced, having the initial torque set to the extremes also voids nearly all benefit from the acceleration and braking sensitivity values, so unless your aim is either the full open or full locked (and it may be in some cases, I lock my diff for drift cars), you should avoid those initial torque values.