If your refering to the 75 thats listed in your sig then you would have to leave it unattended for well over a week before you would notice any significant SG
changes due to evaportaion. On a take that size you should expect about 1 gal
of evaporation per day so that would be about 7 gal
per week. As has been mentioned 1.025 is right on the money and is where you should strive to be. As the water evaporates out of the tank the salt and other elements remain and thus the less water you have the more consentrated these will be. It will take a decent level of evaporation before your SG
gets so high its not managable and simply replacing the evaporated water with fresh (unsalted) water your SG
will fall back in line at 1.025.