We have an aqualogic P4 that shows us the salt level (granted it might take a couple of hours for the salt level to update) should we still get a salt tester kit?
I would get an electronic salt tester. I just did so after a recommendation from a pool guy (that did our pool inspection). They run $100 or so from Amazon. I got the ExTech ExStik II. Tests salt, TDS, and temp.
I found my SWG was off by several hundred ppm after getting the tester. You can also buy a calibration solution for $10-15 (for a lifetime supply) and calibrate your tester every 6 months.
So then, how quickly should I be losing salt? Very often we will get salt up to 3200, and within a couple of days back down to 2700. According to the SWG readings.