The simplest definition would be "however much money is needed for the conditions described in the Universal Declaration of Human Rights to be met". "Everyone has the right to a standard of living adequate for the health and well-being of himself and of his family, including food, clothing, housing and medical care and necessary social services, and the right to security in the event of unemployment, sickness, disability, widowhood, old age or other lack of livelihood in circumstances beyond his control."
I can't answer your question about incentive. To me, it's a matter of right and wrong. And again, I think it's an issue of measure; how much is too much? I don't have a definite answer to that, but to me while the debate of that point is legitimate, there is a kind of obscenity in rich people fretting that their wealth is being 'stolen' from them when others don't have the means to reach the standards of "adequate living".
You're assimilating 'involuntary charity' to servitude, which I disagree with; assuming it's a law in a democratic country, laws aren't about servitude or slavery. They are the expression of the will of the people through their elected deputies, and as such I can't see how the words servitude or slavery apply.