Take off about 2 inches because the brine will pull down to about the middle of the air check valve. Then divide your computed volume by 2.5 to allow for the salt. So about 28.8 gallons. Then multiply by 3 which would be about 86 pounds of salt. divide by 4 cuft, and you get 21.5 lbs of salt per cubic ft of resin. (if I calculated right)No. I have measured the tank and the level of water after it fills and the space it fills would contain 76 gallons if there was no salt present. The tank is 26 inches in diameter and 52 inches tall. Water fills to 36 inches which is roughly 6 inches below the overflow. Doing the math tells me it contains 76 gallons of water and salt, plus the salt in the tank that is above the waterline.
So almost 3 times as much as you should want. So your problems don't appear to be related to insufficient brine fill.