Hello, thank you two for your answers.
I tried to change the font-size and it seems it doesn't scale on it. For the last two pages I put a really big font and my stats change from 1 page per minute to 1.4 page per minute.

So I'm really not sure about it.
It's ok if I stick to a font and a size, but I like to see different font in my differents book
Anyway thank you