You'll need to adjust to what works best for you. Those of us who were guinea pigs before we developed the course varied in our preferred distances, but for the standard letter size diagram we were probably about six to eight feet away.
We also experimented with half page and quarter page sizes that we used at our desks. I experimented with putting them on cardstock and pasting them to my monitor. I probably rolled my chair about three to four feet away for the smaller sizes.
Once you start working with the diagrams, you'll probably pretty quickly determine at what distance your perceptions are most enhanced.
Enjoy - it's a fun process!