At just a quick glance, since the backing is over twice as wide as the regular width fabric, at the most, you'd use half of what you normally need.
So if you needed 3 yards of 45" (42", 41", etc.) fabric, then half of that is 1.5 yards. However, some did the calculation for you, so you have it. But as a general rule, just think "half as much" is needed--knowing you'd have some left over.