Yuku

Kart gave a beautiful answer. In my case, I never use any of these. To level, I just use the focus points in the viewfinder. Even without those grids, you can use these points to approximate the composition naturally. When it comes to leading lines photography or symmetry, it is quick and easy to capture the moment without any distractions. The best time to use such guidelines would be during the post production.

How come no one mentioned this?

I think what you meant was ‘effusion rate’. Graham’s law clearly states that it is applicable only for effusion – “the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles.”