Why are ratios and proportions important in pastry making?

Making good pastry is a science. Basic pie crust, for example, calls for only four ingredients: fat/shortening, flour, water and salt, but they must be combined and handled in a particular way to produce a good result. The right quantities, the right order and the right temperatures must be observed. Too much fat and the dough will be gummy; too much flour and it will be crumbly. Too much water at too warm a temperature will yield a "slippery" dough that will be hard to roll. Any of these actions, along with overmixing and handling, prevent a crust from being light and flaky. Pastry making is not difficult, but it requires following a reliable recipe to the letter. It may take a couple of tries to get it right, but this skill is not beyond the reach of the average cook.