100% would mean all salt, no water!
Or , it could mean 100% saturated !
I think Gurkans recipe is close enough for a brine.
One method is percentage by weight , the other is percentage by saturation.
For the saturation method , Technically , you would have to start with a saturated solution (which is considered 100% saturated) and adjust from there:
You want to heat up your water substantially above room temperature, but it's not necessary to boil it. After the water is fairly hot, turn off the fire and start adding salt with a teaspoon. Stir the water with a wooden spoon as you sprinkle in the salt. Keep on observing the water as you stir. When the water approaches 100% saturation, the salt crystals will no longer dissolve in the water and you'll see the crystals swirling around in the water. Let it sit for a few seconds and then check to see if the crystal can still be seen. If they have dissolved, then add a bit more salt until they remain as un dissolved crystals sitting at the bottom of the pan. Now the solution is 100% saturated and can't dissolve any more salt crystals.
If you measure out exactly one cup of the 100% master salt solution and add exactly one cup of distilled water to it, you now have a 50% solution of salt water. By adding the same amount of plain water to the 100% master salt solution, you have cut its concentration in half (1/2 = 0.5 = 50%). If you take 1/4 cup of the master salt solution and add 3/4 cup of distilled water to it, you now have a 25% salt water solution (1/4 =0.25 = 25%).
Using a base of "100", it's relatively easy to measure out the exact percentage of salt water concentration that you want.