Wouldn't the soundcard in your laptop need to be calibrated to provide an accurate measurement? I know that some room measuring software, such as REW, requires that your soundcard be calibrated in order to provide an accurate measurement.
No need to calibrate the soundcard as you are only measuring a single frequency at 1KHz. I would assume that input into a soundcard is pretty balanced as far as sensitivity goes. If you want to make sure the measurement does not depend on balance, switch channels and see whether the results are the same.
For frequency response measurements, a calibration is useful as most likely a soundcard is not perfectly flat with respect to frequency response.
Good luck,
Rene