It’s not wrong, it’s close enough. And the point it works with more numbers and more type of calculation. Let’s calculate 4% of 1243. That’s the same as 1243% of 4, right, much easier to calculate by simply changing the 2 numbers… While my method is the same, by simply rounding everything.
And in engineering you always multiply/divide your results by a 1.5 or 1.25 safety factor, depending on situation. So you don’t have to calculate exact results, just close enough. E.g. G is always 10m/s2. π is only 3.14, the other digits doesn’t matter.
Yes, in elevators usually one cable could hold far more than the full weight, then they add 5 more for the safety.
For rail speed limits this is the exact way they calculate it. For road speed limits they consider braking distance, which grows by the square of your speed, so if you go 120 on 60 road, you will need 4 times the distance to stop. I wrote 1.5 as a safety factor, not 4, With a 1.5 safety factor you can go by 75 though, but I would use a 1.1 safety there, as in my country the speed cameras are set up that way, you can go +10% of the official speed limit, they only send a cheque if you went even quicker than that.