Rant from a reader of self-help books: Authors, don't do this.
Argh, I hate it when I read a healthy sample of a self-help book for the Kindle, like it, then buy the book--and the author continues for another third of the book trying to convince me how useful said book will be and how much I need their method of doing whatever it is. Look, if I'm able to get that far in the book, it's because I already bought the book.
Dear Self-Help Authors: I've already handed over my money. You don't need to convince me anymore. You just need to give me the information you promised in the sample.
That's right, the sample. Ask your publisher how much of the book they'll offer as a Kindle sample. This is becoming a very useful piece of information. It can and should affect what you write and how you organize the book. Yes, you'll have to consider how much front matter you have (title page, acknowledgments, table of contents, etc.), so your guess won't be exact, but you should still be aware of a reader's patience (or lack thereof).
If you self-publish, the sample will be 10% at Amazon and B&N, no more and no less. You can offer a different amount through Smashwords, but the bulk of your readers will come from the first two.
Heck, Mr./Ms. Author of the more traditional mindset, even if I were reading a print copy, do you think I'm going to stand around in the book store and peruse a full third of the book before making a decision?
Just get on with it, folks. No need to pad the page count--or waste the reader's time, which is likely as precious to her as her money.
Thank you in advance for being efficient.