I'm not a math Major lol. I just know that's what the original framework for the proposals was centered around and after compromises we got what we did, which I agree, is way less then is needed.
To be honest, the 15 an hour is even smaller then most places estimated minimum wage required to live above the poverty line. Each area has it's own estimate, but the average now is (based on gdp growth, rental costs and food of areas on average) up to $22/hr.
So, if I were negotiating the plan, I would have tried to go for at least 50k, which I think is still less then $22/hr... It's kind of a no win situation in the debate until people can see that it actually works which will take a while for people to get over their initial bias and propaganda feed.
RE: Basic income is about efficiently eradicating poverty. So if Bernie Sanders is right then minimum wage should be about $15 USD per hour, or about $30,000 per year. A lot of basic income advocates state basic income should be about $12,000 per year.