Whenever I can control sample size, I set it to 100%.
Honestly, if someone is going to pirate me, a 90% sample size isn't going to stop them.* If someone isn't sure they want to buy me after a 20% sample, well KEEP GOING, GOOD SIR! MAYHAP CHAPTER SEVEN WILT CHANGE THY MIND??
Plus, my mother-in-law always reads the ending first to see if she wants to buy a book.**
* Mind you, I'm very much in FAVOR of being pirated.
** And I have a potentially-spoilery "trigger warning" section at the very back.