That all sounds good. By a pinch I really mean a pinch - it's more of a gentle preventative, and more salt than that isn't good for them unless you're trying specifically to treat something. Fin rot is best treated with clean, warm water and 100% water changes every day until it's gone.
Lucky you with well water. No chlorine! You might want to pick up water conditioner anyway, if you have the chance and money - a good conditioner also removes heavy metals. Be sure to read the label first to make sure it does that.
Bettas don't really enjoy 100% water changes, but in the case of illness or very small containers, it's the best. When you don't remove water 100%, it becomes a sneaky slow build-up of ammonia, because the leftover water has ammonia in it, and it blends with the clean water you're putting in. Every water change, the fresh water has less and less of an edge over the dirty water already present.
When you do 100% water changes, you clean everything, including the gravel, temperature match the water as best as you can, and then float your betta in the new tank. Acclimate him as though he is going into a brand new tank for the very first time by letting him adjust to the temperature, and then slowly replacing his cup water with the tank water over the course of around 30 minutes. If you do so gently, they will have a minimum of stress.
If that bowl is 2.5 gallons, after the fin rot is gone you can go to one 50% and one 100% change per week. That should keep the water clean enough for his continued good health.
