As long as nothing is in the tank, you can just put more salt in. If there is something in the tank, then yes you would want to mix up some 1.027 and when you do your daily or whenever you top-off, use that water.
On another note, I would ditch the hydrometer, as they have a tendency to be very inaccurate. Pick up a refractometer, should be between $40-50 bucks at your lfs. Specific gravity is something that you want to have a very accurate reading on.