Low salinity during cycle

Hi there, i just set up my marine tank yesterday, after letting the cloudy water settle down and beginning the cycle process i thought i would check the salinity of the water(was bought pre made from my fish shop) only to find it is at 1.018 and not at the preferred 1.026, i calibrated my refractometer with Rodi water and am still getting the same reading. i have know fish in the tank at the moment, just live sand live rock a heater and a power head, can i slowly add marine salt directly into the tank???if not what would be the best cause of action. and howmuch salt should i add a time, a tea spoon or so???



thanks



pujan96