Fair point. Obviously an easy definition would be one that's fuzzy and increases with distance above the mean... very much like we have now for income tax, but instead for assets held.
Do you realize that the more rich people leave, the fewer rich people there'll be left to plunder?
That, in turn, means that the remaining rich people would have to be taxed even harder to compensate for some of them leaving, which would then motivate even more of them to leave, and there you have a feedback loop that leads to the Basic Income party being over.