I don't think that they published a torque spec for the backing plate bolts. If you use a medium length ratchet with a wrench holding the nut pull it down and snug it up. Don't pull it so tight that the bolt starts to dig into the metal.
If you want to practice getting the feel find a piece of scrap metal and put a hole in it for different size bolts. Then put a bolt and nut through and give it a try. Use a breaker bar and see if you can shear the bolt off. You will soon get a "feel" for what tight is.