Your interaction with the program is through the sliders. There are two sliders, one for specific gravity, and another for temperature. As you move them, the calculated salinity will vary. Notice that temperature has a dramatic effect on salinity, demonstrating clearly how specific gravity can be very misleading if used alone. The Help panel gives some extra explanations and includes a link to my Brackish Water Aquarium FAQ.
Estimating the salinity of water for an aquarium by weighing out marine salt Mix is not 100% reliable. Salt absorbs moisture from the air, so once the box is opened for a while, the weight of a given portion of salt becomes misleading because some of it comes from water, not salt. Always use a hydrometer or refractometer to confirm the specific gravity afterwards.
Brack Calc 4 is a more accurate approach to the concept that begun with Brack Calc, a HyperCard stack and application designed solely for the Mac OS.
