
You are right - we made a mistake! Fortunately, you and other alert readers caught it, so we've corrected the problem in later printings of the book.
Here are two suggestions: First, you could elevate your legs by rolling up a towel and placing it under your knees as you do the exercise. If that doesn't help, purchase a firm foam cushion to raise the chair seat; you can use a towel with the cushion. And of course, you could buy a new and higher chair.