If we're talking about the fair share in the current system, then we need to do an awful lot of shaving off the top. I don't believe any one person is worth millions, let alone billions, whilst people who are just as capable and probably more in touch with actual life struggle along on a low-five figure wage, if that. A top-heavy graded taxation system that doesn't stop at 100,000 pounds like our ludicrous British system does. Let's say that someone earning, or 'worth' (whatever this strange, unquanitifiable mechanism is), 1 million dollars/pounds/euros/whatever gets taxed 90% for starters, and 60% for 100,000, which for me is still pretty bleedin' generous.
The logistics and mechanisms for such a system would be complex, and the idea of actually implementing it is a pipe-dream, but hey a guy can wish.