At that point, you're restructuring the entire economy, and no one has any idea what that even looks like.So there is currently 16.01 trillion dollars per year of income paid out in the United States (2016 numbers).
There is 230 million adults.
For $12000 UBI a year, it would need 2.760 trillion of that pie. If we were to do a fair flat tax rate against everyone to redistribute that 1 to 1 - that would be a flat tax rate of 18% across all that 16.01 trillion dollars. So essentially the cross-over point of where you would start "losing money" is around $67000 a year. I think it's fair to ask everyone above that income to pay a bit more towards society.
Now I couldn't find exact stats for capital gains, but those should be included and currently it's almost a 1 to 1 for tax revenue between income and capital gains. So assuming that would follow same rule, and ideally a bit more - that would make the break or cross-over point of where you would start "losing money" to ~$130000 a year. Now that even becomes an easier number to swallow and a easier bracket of people to expect to pay a bit more.
So you can easily do a fair flat rate against everyone - no exceptions - just whatever earned from income and capital gains of flat 9% (divided 18% in half if we can also tax capital gains that) across everyone is divided out by 12 and paid to all 230 million adults.
This is the definition of uncharted territory.