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).