[jameshart]

Probabilities? Why do you need to bring those in?

Percentages are just a way of writing rational numbers. Bread recipes are expressed effectively as 1 part flour to n parts of each ingredient. But since n in that formulation is usually a value less than 1, expressing that number as a percentage is convenient. Percentage notation seems completely appropriate for this usecase.

So 60% hydration means 1 part flour to 60% of 1 part water, i.e. to .6 parts water.