I'd go with 20% as upper class. I think of "wealthy" as having money that lets you come and go as you please, just buy a fancy car if you want without really having to think about the finances of it.

There is a D&D-type game that measures wealth as a rating of 0 to 5, and you can make essentially unlimited purchases of items costing up to 1 below your wealth rating essentially at-will. So someone can buy a sandwich whenever, someone else could take a decent vacation/cruise whenever, another could buy a decent car without worry, one could buy a nice house like it's nothing, and finally someone who could buy a mansion or private jet without real concern. Those in the couple-hundred-million to billions range.

I'd draw the Wealthy line somewhere in the mid-4 range on that scale. You could also consider it as "the point where safe/moderate investments could continue supplying a family plenty of comfort without working for two+ generations".