1. The salt level has nothing to do with the amount of softening. All you have to do is make sure the salt is always above the water level in the tank. I.e., not water showing when you open the cover.
2. The amount of softening is fixed so there is no adjustment to it, unless the softener has a "mixing valve" which dilutes the soft water with hard water.
3. The only adjustment normally is to set if for the hardness of your water so the metering valve will know when it has reached its optimum capacity and should be regenerated.