You don’t have to lower UBI by income. Tax does it for you.
Pulling numbers out my arse, you band your tax until an income of 100k means they pay 12k in tax, essentially reclaiming the 1k/pcm they are paid by ubi. All while insuring they are never worse off than taking no pay rise, as they still have 88k to spend on luxuries.
Numbers subject to bitter argument.
You aren’t? The 1kpcm is tax free. It just if you earn more than 10kpcm your tax rate is higher than 1k.