if you follow the pound per gallon i think you should have little enough water to make up for the lack of salt.
1# of salt per 2 gallons of water
1 pint to a pound
1 cups per pint
1 cup per gallons
should e4qual out if the rock takes up 20% of the tank volume or more, so you should be fine.
i know this may not make sense, but if it helps any i've had a few, so my thinings a little skewed, but i understand the question and i think you're fine for now. but if you need to do a WC you will need more salt.