Is there any evidence (beyond anecdotal, like Steven Seagal and Gérard Depardieu) for Rich People Leaving? I'm absolutely certain you can find one or two Rich People leaving jurisdictions after tax increases, but do enough people leave to make a statistical difference?
Not a direct answer to your question, but I think tax rates in Britain in the 1960s and 1970s were very high, then when the rates were lowered, tax revenue actually went up. The term "Laffer curve" was used a lot in public discussion of this topic in the 1970s and 1980s.
Tax revenue hasn't gone up in the US despite many rounds of lowering taxes since the 1980s. Clearly, the US is on the part of the Laffer curve with a positive slope.
Laffer curve revenue increases have to derive from an increase in the marginal benefit of working. I can't see a mechanism for rich people to increase tax revenue if the rates drop.
This video discusses circumstances under which increasing taxes on millionaires+ wouldn't necessarily reduce some counties' tax revenues (and in some cases would increase them). Keep in mind that it does not suggest how to find optimal or fair tax rates.