Zip lock bag.
Hydrometer in bag.
Salt slurry in plastic dish next to the hydrometer.
Leave over night.
Should read 75%.
Not to be argumentative, but overnight may not be necessary. It more depends on the amount of slurry, the wetness of the slurry, and how quickly the hygrometer stabilizes. I just check the reading every half hour or so, and watch for it to be consistent. If you happen to use a little too much salt, and the slurry dries out overnight, then the reading could be too low. So watching it over the course of a few hours can give a better indication.