You're all overthinking it..
So: you have a measured amount of water at 10g salt per litre (or 1%) (this is the right level of salt in pasta water which Italians use)..
So I have 2500ml of water with 300g pasta (hungry boy portion) 28g of salt - so (as 2500ml water weighs 2500g plus 300g pasta = 2800g total volume) then the whole contents of the pot is 1% salt
All I do then is weigh the remaining water after the pasta is drained, then whatever water the pasta has absorbed it has taken in exactly ''x''grams of salt - its a precise enough measurement (for cooking ''taste'' and probably monitoring sodium anyway) - obviously it isn't model perfect for the isolated chemical sodium chloride, especially if you're using rock salt which can actually be low in sodium, (even so you can still be fairly exact by working out the percentage of SALT by total volume of water and pasta) it is precise for ''saltiness'' and in fact pretty precise for common table salt which is just sodium chloride mixed with a miniscule amount of sodium ferrocyanide as an anti-caking agent - in other words it is a good ''real World'' method of working out how much salt you are consuming!
Simples - I had to work it out for cooking up maize for fishing bait - I have to have the bait preserved in the sun on long camping sessions but not powerfully salty enough to make it too tough and shrivel up (osmosis effect) - maize is of course rock hard so is also soaked in the water overnight prior to cooking, after cooking ends up relatively soft like pasta - but then I add enough extra salt to bring the whole container of drained maize up to the saltiness of sea water (about 3.5% salt), and its thus preserved enough with ''shelf life''
I didn't want to get complicated, and have to get on - but God only knows how much sodium chloride is in sea water, or the flash rock salt I use, if the pasta/maize is reacting with and breaking down the sodium chloride itself etc. -- that will be for a scientist to answer, I'm not bothered as life is too short, and its worked for me for years as a trained chef and experienced angler