The Nachi course makes a big deal of a supposed difference between slope and pitch, and muddies the water with examples in different units.
A ratio is not in inches, feet or anything else.
Your 8 feet over 24 feet can be expressed as 8:24 or 4:12 or 1/3 pitch. It’s all exactly the same:
8 ÷ 24 = .333333…
4 ÷ 12 = .333333…
1 ÷ 3 = .333333…
8:24 = 1/3 = 4:12 pitch = 4/12 pitch = 18.43 degrees = 4 & 12 or just “4 pitch”.
To say it again, a ratio (the relationship of two numbers) is not inches, not feet, it does not have a unit at all.
See also Slope vs pitch confusion in roof inspection training
2021 IRC defines “Pitch” as “See Slope".