I've asked that question of five or six Apple/IOS developers, some of them well known, and they've all just laughed at me.

Worrying about coming close to $1mm/annual sales is something precisely zero of them were worried about.

The answer to your question is, according to what I've read, yes - at $999,999 you owe $150K. At $1,000,001, you owe $300K. But - if you can get to $1mm, it's highly likely that $1.5mm, $2.0mm+, etc... are coming soon enough.

Many developers are focussing on $50k, $60k, type scenarios, where the 30% to 15% means enough money in their pocket to afford new development hardware, etc...