At the end of your sample -
It's not as impossible as it seems. Use the previewer positioning to work out roughly where you think the ten percent mark is going to fall. If need be, and some of you artesans are going to be appalled, adjust your manuscript after you see the book live.
Don't be having someone getting to the end of your sample and find it petering out.
"Julia walked her dog through the park, just as she did every morning. It was the one time when she felt she was at one with the world, just after dawn when only like-minded people were to be seen. The only time of day when a stranger was likely to greet you in New York."
"This was dangerous, and tempting. That guy she'd seen every morning for the last two months had just walked over and propositioned Julia. Right here, right now. And if she agreed, it would be behind those bushes. What if someone seen them? where would she tie the dog?"
The artesan's among you will baulk as you have lovingly crafted your masterpiece. Fine, and if it's not selling cos your sample ends up with Julia walking the dog, up to you.
In my case, I'd want to know where Julia tied the dog. Such questions would induce me to actually buy the book.