Matteo, I'm certainly not as knowledgeable as our instructors and masters here on the forum and within the SF community but I'll offer you this. Just start it and follow the way it's written. In week one, start with 3x3 C&P ladders for the heavy day and each week add one until you reach five ladders. Then work on rung increases. That's how I approached it.
Additionally, on the "light day", when doing snatches, don't be tempted to go all out. I think the book calls for 50% of what you could do. I learned the hard way with ultra sore shoulders for the first few weeks. By the end of the program, your shoulders will be conditioned for a lot of volume and you will look and feel strong. It's an awesome program!