i would mix a bucket full of saltwater. I use a 5g bucket and put 3 cups of salt in. i let it sit for a few hours and test it with the hydrometer. After awhile you will be able to known who much salt to put in to get the desired SG (still check with your hydrometer just incase).
Take your starting SG and multiply by 5, then add the SG level of your replacement water. Then divide by 6 which is 1/6 of your tank volume. Then you get your approx SG. Hope this helps.
