This is how you calculate. For instance, let's say you do a five percent water change with just freshwater.
1.030(.95) + 1.000(.05) = 1.0285
Alright, lets try a ten percent change with freshwater.
1.030(.90) + 1.000(.10) = 1.027
Still not enough, how about twenty percent with freshwater.
1.030(.80) + 1.000(.20) = 1.024
As you can see, you will need to take out about 20% of your tanks volume and replace with freshwater to attain a specific gravity of 1.024. You do not want to do this all at once. Taking out 5% and replacing with freshwater lowers the SG
about .0015 (as shown in the first calculation). I would take out about 5% of the water and replace with freshwater, everyday for about 4 days. (you may actually need to do this for five days, because we did not account for evaporation, which is constantly raising the SG