I think that depending on who you are talking about you will come up with different variations of where point B would be.
For example, I think it depends on what form of government you are talking about. For example, even in Communist societies tax rates were basically 100% and people did still work to some extent.
Here is America, I would argue that the lower income brackets could handle a tax rate depending on what social services where then provided. The rich I think would have a different point "B" because for that group, getting taxed at a higher rate makes them worse off because they will not receive many of the social programs that the poor do.
I think the Laffer Curve has its share of problems, and will vary from economy to economy and in fact person to person. Every person will have a different level of tolerance for how much the government takes.
For example, I think it is ridiculous that I am taxed at 35%, whereas my brother, would argue that he is happy to live in a country that affords him the opportunity to earn as much as we do. Therefore, we would have a different point "B".
Supply Side Economics focuses on giving people more incentive to produce things. This will come in the from of tax cuts, which will grow the economy and cause this economic growth to offset the loss of tax revenue. It is a basic theory, but it is still debated as to effectiveness. I am sure you can google it if you want a better definition than what I just gave.