I could not locate any source that discusses the salt to water ratio. the idea behind the salt water is that it symbolizes the tears that the Israelites shed from the suffering of the hard work.
If you wanted to adhere to this symbolism, I guess the amount of salt should be equivalent to the saline concentration of human tears. I'm no chemist, so I have no clue as to what this is.
However, otherwise, there doesn't appear to be any particular amount of salt needed, as long as one can taste the salt. Of course, for health reasons, you want to or need to limit it for you or guests, anyway. As far as I know, there is no problem with using the same bowl of salt water, unless you are squeamish about "double dipping".
Just a note: dipping the parsley (or whatever karpas you use) is a Seder requirement. Eating a hard-boiled egg is not a Seder requirement and is simply a custom.