[coldtea]

Also one could remove the UBI part already covered by pensions -- e.g. the millions of government employees getting X pension money, can get (X - UBI) + UBI (so the same as they already get).

[dragonwriter]

That's not a UBI, then, because it's not unconditional.

Also, I suspect the formula you intend is X + max(UBI - X, 0)—or, more simply max(UBI, X). Consider what your formula does for pension < UBI.