I did do the research about a year ago, but I don't have all the numbers at hand anymore.
UBI of $2000/month per adult, $800/month per child, flat tax of about 45-50% on all income, pretty much no deductions, no taxes on capital gains or interest/dividend income (but no deduction on interest/dividend payments or capital losses), elimination of gift/estate taxes, a VAT of about 25%, instead of deducting charitable contributions the organization gets a percentage of all contributions in additional funds directly from the government, eliminate welfare/SNAP, eliminate minimum wage. Single-payer universal healthcare would be available.
If we want to continue to subsidize certain things like home loan interest, they'd be direct reductions in the interest rate rather than deductions from your taxable income. All income, except the UBI itself, would be subject to the flat tax, paid directly by the employer.
Corporate taxes would be at the same rate as the personal income tax rate, with only direct costs deductible (not business lunches or advertising or corporate jets except to the extent they can be shown to actually save money over alternative transportation). This is where capital gains and dividend payments are taxed. Depreciation of actual working assets would be allowed as ongoing expenses as long as any resale of those assets is counted as income.
The income tax (personal and corporate) would be automatically set to provide 50% of the annual budget needs, while the VAT would provide the other 50% (based on the previous two-year period's numbers or similar).
Most individuals would never need to file a tax return. Payments would all be electronic to save on costs to administer.
Eliminating capital gains and dividend income is reasonable because you're collecting the taxes through a different route - and basing the country's budget and economy on the vagaries of the stock market is insane. Taxing everything at the source eliminates most ways of avoiding taxes. If a business is paying someone under the table to avoid taxes, they are just going to be paying a higher tax themselves since those payments won't be legitimate business expenses. Etc.
Yeah, living on $24000/year for a single person might not be great, but it would give people the freedom to move to places where prices are lower without worrying about whether there will be jobs there to support them. Once they move there, of course, then more jobs will become available as the economy picks up in the low-priced areas.
A UBI turns a flat tax into a progressive tax. UBI of $24,000 and flat tax of 50% means someone with income of $48,000 is paying 0% tax, $100,000 is paying 26% tax, a couple earning $120,000 total is paying 12%, a couple with two kidswith $250,000 total income is 30%, at $1,000,000 for one person the effective rate is 48%.