The book says nothing about the strength of cayanne used and from what I have read many people just use the 40 as it is readily available. I am definitely aware of the added benefits of using a stronger form, however.
I plan on starting with the 40 (but I use a little more than the recipe calls for). As I progress I will increase the strength of the cayenne in the lemonade. As this is my first Master-Cleanse I am still getting used to the taste/effect and don't feel the need to push things too much past the simple outline in the book.