The neti pot instructions provided with my pot say 1 tsp salt (preferably not with iodine added) or 7/8 tsp salt + 1/8 tsp baking soda.

That's for 150 ml (around 2/3 of a cup) lukewarm water.

I've just been using it for a week but I think it's a bit on the strong side. I'm putting a bit less salt now and it's better.