If no one works no one pays income taxes; this is one reason why I mentioned sales taxes in another comment. Income taxes are complicated to administer and a bad incentive structure anyway.
Traditionally, sales taxes have been considered regressive because they are linear; you can't bracket them like income taxes, so they heavily burden the most poor. But a UBI is progressive enough that a flat sales tax to fund it should still leave society more equal.
The UBI check goes out every month; the sales tax percentage is adjusted until the revenue matches the amount sent out as UBI (or more optimistically, adjusted until inflation is kept to a stable target).
Let's say that the sales tax reaches as high as 40%.
This means that if you are unemployed and destitute and the UBI represents your entire spending power, ~40% of your UBI check goes back to the government when you spend it. If you are a rich guy living the good life on your property portfolio rental income and MSFT dividends, 40% of your spending also goes back to the government, which is many multiples of your UBI check (which constitutes a trivial amount of your income).
Remember, the UBI doesn't amount to all gross income, even if nobody is employed. When you spend money to buy furniture, somebody gets it as their income, even if it's the shareholders of the IKEA robofactory. The people on the top will end up paying more into the system than they get out.
So you’re assuming rich guys will spend so much that 40% of their spent money will cover 60% of the entire UBI budget? Or do you also want rich guys to pay income tax? No matter how I look at this I don’t see it working without a massive forced wealth redistribution from rich to poor.
It's a mathematical certainty. Let me introduce it another way: the government imposes a 40% sales tax and then distributes the resulting revenue evenly as UBI, however much it is.
Do you still have concerns about the program not covering the UBI budget?
Aside from that, wealth follows a power law, so the rich guys do spend a lot. They have some very fancy champagne. I also elsewhere mentioned that the consumption value of land ownership (the equivalent rent) should be included in the sales tax, so that will be quite a substantial redistributionary force.
Traditionally, sales taxes have been considered regressive because they are linear; you can't bracket them like income taxes, so they heavily burden the most poor. But a UBI is progressive enough that a flat sales tax to fund it should still leave society more equal.