The biggest help for me has just been to be specific as to how they write. I might specify block letters or write something on the billet to give them reference.
Personally I don't do any billet work or book tests with much more than a word. In my routine I use a dictionary so it is more natural to just chose a word rather than a phrase or sentence.
Lastly, you might have to work around the restrictions of your eyesight, if that is an issue. You might consider forcing a book page rather than peeking it. Or getting one ahead with your billet peeks, this may help to change the moment a little bit.