1000 ppm in 12000 gallons is 100 lbs of salt. But I would add half at a time to see how much it moves up just to make sure there isn't a non-linearity in the salt reading.

Did the pool store use test strips to measure salt? I have found those to read fairly high compared to a drop kit or the SWG.

The bottom line is that you have to make the SWG happy so go by what the SWG is indicating. A failing cell will tend to read a low salt level so keep an eye on the SWG reading and if it continues to drop, it might mean a failing cell. Also, you might try and clean the cell anyway just to make sure there isn't any scale.