I believe 24" diameter tanks are 7 cubic feet of resin per tank. You want a salt dosage of 10-12 lbs of salt per cubic foot. So 70-84# per regeneration. need to adjust your refill accordingly. You need 28 gallons of water to dissolve 84 #'s. If your refill piston is 2.0 gpm it needs to refill for 14 min. NOW THE other adjustment you need to make is to your gallons per regen frequency. Each tank should be able to treat 21,000 gallons of water conservatively each regeneration cycle. Age of unit, iron, manganese, chlorine can all factor in but this should be pretty close to getting the most out of your softeners.
P.S. I am a Culligan Water Dealer who just bought my first car wash 6 months ago so i feel only fair to share my knowledge since i have gained so much from this forum. Thanks