Haha, @Nico, That’s one way of checking the curve, lay your ruler on the screen, zoom in to fit and then fiddle with the handles to get it to match. I have problems taking the thin rullers off the screen 
They seem to attach themselves in such a way that nothing fits between the screen and the ruler to pick it up again.
These various curve rulers seem to follow the Fibonacci rule as far as actually creating the curves to follow from a smaller curve into a larger curve smoothly.
Here are 2 methods one can use to get this smooth effect that may help you:
Perhaps one of these to methods will help you.