Salt-water pools are NOT chlorine free pools. A salt-water pool is simply one that utilizes a chlorine generator.
Salt water chlorination is a process that uses dissolved salt as a store for the chlorination system. In order to achieve this, the chlorine generator (also known as salt cell, salt generator, salt chlorinator) uses electrolysis to break down salt (NaCl). The resulting chemical reaction eventually produces hypochlorous acid (HCIO), and sodium hypochlorite (NaClO), which are the sanitizing agents commonly used in swimming pools. As such, a saltwater pool is not actually chlorine-free: it simply utilizes a chlorine generator instead of direct addition of chlorine.
When adding salt to your pool: If you think you need 6 bags according to your calculations, add 5 first. Let it dissolve and retest your salt level before adding your last bag. It's a lot easier to add another bag if required than to have to reduce the salt level if you've overshot your target salt level. A high salt level above 4500 ppm can rapidly reduce the longevity of the cell.