I like the water to at least be as salty as the sea. I think it depends on how big and how full of water your pot is. From my experience, I've learned that I like a lot of salt in this process. This is how you season your mozzarella, so you can be generous. I'm sorry I don't have a more specific answer for you, I think this is more of a personal preference.

You can also knead some fine salt in during the stretching process, so you may want to err on the side of less salty water if you're concerned.