The amount of salt storage vs. salt dissolved are totally different. Your salt tank needs to be large enough to hold 3-4 gallons of water above the air check in the bottom of the brine tank, the salt displaces approximately 50% of the space water will occupy. An 18" diameter round brine tank will dissolve approximately 1.6 pounds of salt per inch of water in the salt. A salt grid will allow a smaller brine tank to be used in many applications since it eliminates the displacement of water by the salt as high as the bridge goes.
The maximum water height is determined by how high the float can be set.
In general, even the smallest brine tank will work up to 10 pounds of regenerant. You should be just fine.