I dispute that you couldn't get closer than five or ten percent. Although it was necessary to interpolate between the marked graduations, 1 to 5 percent should be attainable.
However the need to place the decimal point by mental arithmetic opened up the possibility for major miscalculations.
At engineering college we used to refer to them as "guessing sticks".
I still have mine, and showed it to my teenage granddaughter who was totally bewildered by the principle.
Personally I was OK with straightforward arithmetic but when it came to logarithms my brain went into overload. - Also with calculus buts that's another story.
It never actually mattered though, because lots of engineering jobs demand organisational and communication skills rather than mathematical ones.
They don't tell you that at college.
