Federal poverty level is 15930 for a two people family unit, that gives 8000 USD/person/year (slightly below 10k), * 300 mln is 2.4trln. Total income from social security tax (assuming you're replacing all social security with flat just-out-of-poverty basic income) comes to 1.1trln. So definitely short, but assuming 4% annual growth (figure reverse engineered from budget predictions in the futute), that gives us 20 years when in real terms we would be able to fund it entirely from social security tax.
Of course. But you were also talking in nominal (not inflation adjusted) returns when you said that 20 years of growth in Social Security income could fund a basic income.
You can inflation adjust neither or both sides of the equation but not just one which is what you were doing.
4% GDP growth year-on-year while a bit on the wishy side is not THAT optimistic of an assumption. On the other hand, 59% in 20 years of moving poverty line is essentially just accounting for the inflation. It's true that the budget is not inflation adjusted, but I think 3-4% GDP growth on average is achievable (3 would push the limit quite a few years in the future of course)
Yes, but again you're talking about 3-4% nominal GDP growth not real growth (which will be less due to inflation).
If you want to pay for something using nominal growth you have to also accept the nominal growth in the cost. So in 20 years you will be able to fund 15,930 per year but that won't be enough anymore because that number will have risen.
Sources: http://www.usgovernmentrevenue.com/federal_budget_detail