The point of taxing the UBI itself (even before earned income enters the picture) is precisely to adjust the amount in a predetermined way - in this case, it's supposed to be proportional to the fraction of GNP that's not affected by government taxation, so that, as you put it, "everybody is equally affected by taxation and government spending". One issue with this is that it may make the UBI too volatile, which is bad as you want it to be as small as possible on average (because redistribution is very costly, even with the best system you can think of).
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.