The main issue here is that using the same noise level value in different scenes may result in different perceived image quality. What we would like to have is a reliable noise level limit that would guarantee exactly the same final image quality regardless of the rendered scene. With this, we could, for example, agree on specific preset values for the noise limit that would always work, let's say: 2% always means no perceivable noise whatsoever, 5% means fine clearly visible noise, 10% and more means draft quality with heavy noise.
Right?