The volume of water in the salt tank depends on your salt dose lbs. and that varies with different size softeners but more so the amount of hardness etc. in your water and what type of resin is used and how much water is to be used between regenerations.
So no one can tell you without much more info.
And you control the volume of salt you put in the tank, which should not be up to the rim of the tank; a 1/3 to 1/2 full is best and check it more frequently.
Not all softeners have a salt grid/platform in the salt tank and that 1" mentioned, it may not be appropriate to your softener because you may have Pre brining or no grid or a higher salt dose that would cause the water to be much higher or lower than the 1" above the grid.
Do you think there is something wrong with the softener's operation?
BTW, the volume/height of water in the salt tank is not going to be in any manual anywhere.