Good info!

The tomato juice I have lists on its government mandated nutrition statement 680 mg of sodium per 240 ml serving.  One teaspoon of salt contains 6,200 mg sodium chloride, of which 2400 mg are sodium.

I want to make approximately 700 ml of tomato juice.  This would mean I would need to add (240/700) = 2.92, 2.92 x 660 mg sodium = 1986 mg of sodium to add to 700 ml of liquid.  Further to figure total salt  use 1986mg/2400 mg per tsp = 0.83 tsp or 83% of a tsp of salt.

So add just a eyeballed bit more than 3/4 tsp.  Or round it up to a tsp just to make it easy.  An extra 1/4 tsp of salt isn't going to make it overly salty.

Does that sound right?