First you mean salinity/specific gravity not alkalinity.
When the water evaporates salt stays behing therefore raising your salinity. Topping off with the same amount of freshwater that evaporated brings it back to normal salinity. Preferably you want to use RO
What I do is I have a piece of tape in my sump at the water line so I know where the topoff level is. I am now moving toward an automated system however manually works just fine.
That is why I said it is a little better to have it a little less than 1.025 so that if you are not home and water evaporates, the salinity will not raise to high. Just my .2