Baking – What does “spooned and leveled” mean

When making pie crust, you want to a) keep your fat(s) cold and b) avoid over-working the dough, both of which are difficult to do when working in large batches. Cold fat, evenly distributed throughout the dough, will steam and melt away during baking; that leaves the air pockets in the crust that cause flakiness. Developing the gluten in your dough is necessary for a strong, elastic crust, but if you develop too much gluten, your crust will become brittle and tough. Working in small batches allows you to thoroughly combine the flour and fat until you've reached the "pea" stage, and then incorporate your water just enough to bring the dough together for gentle rolling. The best thing you can do to pie crust is to handle it as little as possible.

If you're using a large food processor or stand mixer to "knead" bread dough, you could get away with making more than one or two loaves at a time because you want to develop your gluten to a much greater degree than you do with pie crust. Be sure, however, that you mix/sift together your dry ingredients thoroughly before adding the liquid components of your dough whether you're kneading by hand or machine.

I've always been taught that baking is a science when it is compared to cooking. Cooking is very much 'to taste' and very individual. There are not as many things that can go wrong with a standard recipe in cooking, and you have a lot more room for creativity. You don't have to look at baking as that precise. However, unlike cooking, where you can add or subtract from a recipe with no real harm to physical structure, that does not hold in baking. For examlple, if you feel that there is too much salt in a recipe, cutting back can (and most likely will) have a cascading effect through the ingredient chain. You have to understand your ingredients and the effect they have on other ingredients. That is what makes baking a more precise science.
And in terms of flour, it is often the most 'ranged' ingredient. Depending on flour type, miller, altitude, water temp. etc., the amount of flour in a given recipe is always a guide. Again, you have to know your ingredients. You will never see 'one and an eight cup plus 2 TBSP hard flour' in a recipe, because it is so variable for many different reasons. Flour and water are the two most flexible ingredients, and are always variable. Hopefully this helps, I am sure someone will come along with a more scientific explanation for all different ingredients, I am just offering an experience based answer to your question. Also, you are right, do not mess around with baking soda and powder. If you do, you are asking for trouble. I would suppose that the rule of thumb would be that the more exacting an ingredient is (tsp, quarter tsp), you don't want to change much.