The three easiest ways to give a snail more calcium are to add Kents Liquid Calcium to the water (adding normal dosage to the tank is fine, research was done by a member of another board and no negative effects were found), add crushed coral to either your tank or the substrate (IMO this is the best method because it will keep your pH at proper levels and provide calcium), and feed veggies high in calcium such as kale or spinach. I'm not a fan of adding cuttlebone to a tank, most snails want nothing to do with it and it just dirties up your tank.