What is lighting ratio and how is it calculated?
It's the difference between your main light and fill light expressed as a ratio of brightness. Example: if your main light is twice as bright as your fill light, the ratio is 2:1. A high ratio will give a more contrasty effect, as shadows cast by the main light will still be dark. A lower ratio means the fill light lessens the effect of shadows cast by the main light, giving a flatter effect.
Note a difference of one f-stop is a factor of two, so a ratio of 2:1. Similarly, two stops is 4:1, and three is 8:1.