Some people like to pull the chair back and prop it against the edge of the table. This allows them to look at their foot for photofocus at the same time.
You're allowed to try different positions. Propping it against something makes it easier by taking the weight. Works on bookshelves in the bookshops too.
Then a chair with an armrest is comfortable too. It takes the strain off the shoulder by placing the book on your thigh and resting your arm on the armrest.
I recommend experimenting. Only on the larger books that take more than 5 minutes to PhotoRead do I find it necessary to sit at a table.
Although formal posture is one of the things that help your speed and I strongly recommend it for all the other steps.
Alex