UBI is generally not metered by "%" but some flat quantity of money, whether nominal or real. That is, like a "head tax," but...negative.

In that common formulation, it would compress consumption by the entire tax+benefit base, that is, everyone would move towards median consumption by some amount, keyed to the magnitude of the UBI, if funded by any kind of proportional taxation (including a nominally regressive proportional tax, like consumption tax/VAT).

Politically, it has tough problems: 18% of the population [over age 65] already has a "MeBI" in the form of Social Security that they can vote to increase, and 22% of the population is below the age of 18, and can't vote. So that's 40% right there. Of the remaining 60% in their working years that produce the output split among themselves and that 40%, quite a few would rather not be compressed towards median consumption: the voting population is shifted higher in the consumption deciles, and people are not often so disposed to think they might find themselves luckless in the future. There's a thicket of "tax expenditures" that can form a "MeBI" for the electorate at the upper-half, like the mortgage interest deduction.

If we look at the difficulty in gaining electoral support in splitting consumption to the benefit of minors (thus, future labor) to even things out a bit, in the form of the semi-recently expired expanded child tax credit, we see the magnitude of the political problem.

Personally, I prefer to see UBI as tax reform to avoid crazy wiggling in effective marginal tax rate. But there are many reasons why it's unlikely that the electorate would see it that way, or approve of it even if they did.