if you could know the gh of your local water source before softener, you might be able to guess how much Na is replaced into your drinking water. If the water is not hard, let's say GH12, that is still not much Na replaced into the water.
If you have the water source with GH around 24, then Na into the drinkin water is big. You may also check the salt concumption of your water softener.
There is an argument between some guy and the U.S. water baord in the past for this softener application.