It have been answered
@vangvace I do not know if it is you or me that have misunderstand these 70 %. As I understand it 70 % of the compounds in seawater have its origin from NaCl - there Na is the positive ion and Cl the negative. IMO - this is not the same as to say that Na is responsible for 70 % of the salinity. It is also my opinion that a two part solution will not give an excess of Na ions, on the contrary it will give an excess of chloride ions (if it is not balanced with (as an example) sodium sulfate) and that the original Balling three parts was developed just with a balance of Na and Cl ions as a goal.
Knowing the 4 most important (for salinity) positive ions in your saltwater - it is possible to compute an estimate of your salinity -
as an example with help of this calculator This link will also explain why and how.
The ion director will give you these most important cations - but you would also need the sulfate content too (if you not know your chloride levels) in order to get a good estimate of your salinity.
As an example - from my last Triton Lab:s report I used only the four cations ion that Ion director will give us and get an salinity of 33.77 psu. If I put in my S result too - I get a salinity of 34.50 psu. If I use all that the calculator demands - I get 34.52 psu. In the first example - I had S to 0 - if I put S to the normal 900 ppm (in seawater) - I get 34.39 psu.
My salinity meter read 33.4 psu at the sample time and my calibrated refractometer give me just under 35 psu.
IMO - knowing the four cations that ION director gives us and assume a S content of 900 ppm will give us a very good estimate of our salinity - using the calculator I linked to.
Sincerely Lasse