To get you Nitrates down you can do a couple water changes. A good skimmer will also help with this. As a last resort you could always use a nitrate sponge.
Salt creep is exactly what you saw. From the water evaporating and from power wire going into your tank(heaters, Powerheads) salt will slowly follow up the cord and out of the tank.
As far as measuring salt, just follow the instructions on the bag. NEVER add salt directly to the tank, and try to age it atleast 12-24 Hours before adding it.
A good SG
level is 1.025 so the 1.030 is a little high.
Make all changes slowly.