I think of it more simply in terms of a 100% slope- i.e., a 45° angle. It’s 100%, because the distance you go forward is 100% (or the same) of what you go vertically.
For those familiar with roof pitches, a 12 on 12 roof is a 45° angled roof – 12 inches horizontal for every 12 inches vertical. 12 is 100% of 12.