The Tropic Marin Original Balling Part C is for a 70 : 30 ratio, 70 % sodium chloride and 30 % Part C sodium chloride free sea salt. Starting from sodium chloride only as in your case, you have to add 43 % of the assumed sodium chloride excess in Part C.
Example: You have an excess of 3 g/l of sodium chloride. 3 g x 43 % = 1.3. You have to add 1.3 g per liter or 4.9 g per US-gallon in Part C while reducing salinity of the tank water.
In total the excess of sea salt now would be 3 g/l + 1.3 g/l = 4.3 g/l. This excess is sufficient to supply sea salt (normal dosage for 35 PSU is 39 g/l or 3.9 %) for 0.11 l to achieve ca. 35 PSU salinity.
This means you can remove 11 % of saltwater and s l o w l y add 11 % reverse osmosis water with the calculated amount (4.9 grams per US-gallon water volume) of Part C.
Since you are removing 11 % of the tank water you can subtract this from the Part C also, so take just 4.4 grams of Part C per US-gallon of water volume.
By the way: Very interesting
article on salinity and Goniopora in the BRS video. It seems to confirm my experience that a slightly reduced salinity of 33 PSU is at least as good or better than 35 PSU for corals. In fact in the tropical coasts where most corals grow the salinity is also slightly reduced to ca. 34 PSU due to heavy rainfalls.
