You can use the BleachCalc - follow this thread for information on where to get it: http://www.poolforum.com/pf2/showthread.php?t=4735. It performs a wide variety of chemistry calculations, including those for salt.

For example, according to BleachCalc, for a 10000 gallon pool, adding a 40 lb bag of salt will raise the salt level by 479.9 ppm. Keep in mind that your pool water probably already does contain a couple to a few hundred ppm of salt as well.