Someone who actually knows may chime in
but I think it's very individualized. Not only do people's sweat rates vary hugely, but the amount of salt excreted in sweat varies, too. Even for any given individual, those two things vary over the course of a season with their heat acclimation.
Personally, I had dozens of episodes of mild hyponatremia until I figured out that's what was going on. I find that as with most things food-related, my body will tell me if I let it. The same dish, that I'll have to add more than 1/8 tsp of salt per serving for dinner after a long ride, will taste perfectly fine without added salt as leftovers for lunch the next day.
Unfortunately most people in the developed world are so conditioned by the food industry and the medical-industrial complex to ignore what our bodies are telling us, that it's difficult to learn to listen.
I don't know any method other than trial and error, unless you're willing to get a long series of blood tests pre- and post- exercise. Don't forget that you need to replace calcium and magnesium as well - even if you're already taking a supplement, you need to increase the amount if you're sweating heavily.
Speed comes from what you put behind you. - Judi Ketteler