I played the game 3 times and 3 times it was implied that units have to roll double 1's for break tests when their unit falls to 25% and below.
Yet after re-reading the book, I didn't find that rule at all. The only thing related to 25% if you have lost that much and you have to take a break test.
Though the double 1 rule appears in the 40k book though.
Also, there is no minimum range for shooting, which is something I got wrong in my first WFB game.