You have a sand filter so you will lose some salt and CYA with each backwash. It won't be a lot but that is why it is important to test the salt and CYA every 2 weeks to month until you get a handle on the amount of loss in your pool. Do you have a SWG? If not then you can be a bit more lax on the salt since you don't need it as high s 3000 ppm to enjoy the benefits of a 'salted pool'. Most people who have added salt without a SWG indicate that anything in the neighborhood of 2000 ppm or higher is fine.
I add salt twice a season, once in the spring right when I open the pool and again in early august when the level has come down a bit. If you have dramatic amounts of rain you may need to add salt more frequently.