This particular method is mathematically equivalent to both of them. There's no actual difference between "You get full UBI and you pay income tax on other income" and "You get X cents less UBI per dollar of other income".
The system could be as simple as "UBI, also income tax is 50%, end of description".
The question isn't about mathematics, it's about psychology and politics.
In the second scenario, when do I get/spend my UBI money?
> The question isn't about mathematics, it's about psychology and politics.
That's valid, but when someone's roughly describing a plan I'll give them the benefit of the doubt and worry about whether it can be framed the correct way rather than whether they framed it right.
> In the second scenario, when do I get/spend my UBI money?
I don't know, every two weeks? So it's the difference between a $460 UBI check and a $400 paycheck, versus a $60 UBI check and an $800 paycheck.