I got less than 12% but the fact you'll genuinely away away puts you in far stronger negotiating position.
The best negotiators set their minimum targets and walk away when they're not met. If they let you walk you know it was not achievable.
One key point though is to make sure you've got a salesman and sales manager who know how to negotiate. Many don't actually walk away, they simply give up or don't engage due to skill deficiencies.
I set a reasonable win-win target for a highly desirable and currently easy to sell car which my dealer met with no hesitation, so I set the goal a bit too low. On the other hand he gave me a better trade-in than their first pass so we were both playing the game to get to a deal, which we both wanted