I personally like nice even fifths. The bottom 20% are lower class, next 20% lower-middle, next 20% middle-middle, next 20% upper-middle, and the top 20% as upper class. The middle class, being the bulk of the population, can be those three middle fifths, which leaves quite a spread but focuses nicely on the middle of the bell curve.
So where does a household (solitary individuals, other non-family households and families combined) earning $250K/year sit in that breakdown? In the 2009 tax year (the most current data), there were roughly 2,372,000 households earning $250K+/year out of a total of 117,538,000 households in the US. That puts a household earning exactly $250K/year above 97.98% of the whole batch. That seems quite a lot towards the upper end to me, and not very middle at all.
So how do the fifths break down?
Lowest: $0 - $20,453 (Mean $11,552)
Second: $20,454 - $38,550 (Mean $29,257)
Third: $38,551 - $61,801 (Mean $49,534)
Fourth: $61,802 - $100,000 (Mean $78,694)
Fifth: $100,001+ (Mean $170,844)
And for good measure, the top 5%: $180,001+ (Mean $295,388)
I would imagine that the vast majority of new businesses created are created by people earning substantially less money than you claim. I see a lot of small firms of professionals, construction & trades contractors, mom & pop corner retail stores, basement tech startups, or even just entrepreneurs with a crazy big idea they try to make real. Some may have had a sweet job earning the big bucks before striking out on their own, but I doubt it's anywhere near most. A lot of people mortgage their houses to get startup money, then seek out venture capital as things get rolling. Many fizzle and fail in the first year.
Where you have a point about business creation is for people earning $250K+/year in investment income. They tend to be able to start businesses left and right, and can afford to have a few fail without disrupting their portfolios. But we call those rich people, because they usually have net worths in the multi-millions.