If you have a "pure" UBI that is paid out, and you create 0 taxable value, you receive 100% of UBI.
If you have negative income tax and you create 0 taxable value, how much do you receive?
The same amount you would on UBI. As I said, it's the same thing. Both are a linear function of your income. a*(income)+b
Doesn’t negative income tax without any qualifier imply that b is zero?
No, to have negative income tax you need to set a baseline relative to which income tax is negative. E.g. your income tax is what you earn above 30k, positive or negative. Then if you earn 0 then b>0