Jeff,
I was going to suggest something similar to Michael's idea. Purpose by chapter. Purpose by time period. Whatever way makes sense for you to break things down a bit. Establish a purpose within those confines. It can be overwhelming to try to establish a single purpose.
Actually, that's what is nice about the 5-Day Plan. Since you quickly reach your purpose, you can have a new purpose each day and get more out of the book. Some days you may just refine your purpose. Other days you may establish an entirely new one. Remember that you're doing this in layers. It also makes for nicer mind maps to have more definitive purposes.
I would consider combining the 3-10 books/day and the 5-Day Plan. One process gets you comfortable/competent with the PR step. The other gets you comfortable/competent with other pieces of the system.
