Spot on 
Then just repeat that... add 5 gal with 10 tsp 5 times or so (cuz the math gets more complicated once you are adding to an unsalted tank, diluting the mixture, and then taking from that diluted mixture... 5 times is a good call) with at least 4 hours inbetween. This will get you close to the salt level you want, and then you can do PWCs as normal while replacing the salt you took out.
Then just repeat that... add 5 gal with 10 tsp 5 times or so (cuz the math gets more complicated once you are adding to an unsalted tank, diluting the mixture, and then taking from that diluted mixture... 5 times is a good call) with at least 4 hours inbetween. This will get you close to the salt level you want, and then you can do PWCs as normal while replacing the salt you took out.
