What is the ratio of baking soda to water for lentils?
None. I have recently discovered that lentils do not require to be soaked prior cooking - and nor baking soda. I have already detailed this in an answer but I am not sure how to find it and link to it. I try and I will edit this.

Another point is that the producers recipes were all clear about this : they recommend a minimal amount of salt in the cooking water, the rest to be added /adjusted afterwards. If there is an unwanted effect related to sodium, it might apply to baking soda as well.