Some things, like baking, require everything to be just so. Exactly one tablespoon of baking powder, exactly three cups of flour, exactly a quarter-cup of butter, oven set to exactly 400 degrees Fahrenheit, etc., or everything goes pear-shaped.
Other things, like cooking, are a little more free-wheeling. Casseroles and quiches are particularly flexible. Tonight's example: tuna noodle casserole. We're trying to use up food in the fridge and pantry instead of buying more, so the only item I picked up was dollar-store noodles, since I didn't have quite enough macaroni. (Besides, I like egg noodles for this dish.) The recipe calls for two cans of tuna, and I only had one, so I substituted 12 ounces of canned chicken chunks (thanks, Mom and Dad!). It also calls for a half-cup of milk, but I put in just a little bit more since I had some evaporated milk in the fridge that needed using up. I didn't have red bell pepper, but I had half a green bell pepper, so I threw that in. I had frozen peas, but I also had my eye on some fresh asparagus languishing in the crisper bin. We could have steamed the asparagus and eaten it on the side, but instead I cut the asparagus into small pieces, parboiled it for about a minute and added it to the casserole instead of the peas. And we're out of breadcrumbs, so I crushed up some cornflakes instead, mixed those with butter and sprinkled the crunchy goodness over the top.
Life is like this. Some of it requires precision and fine-tuning, but usually it's more like cooking -- things don't turn out perfectly, you may have to alter your plans or your expectations a little (or a lot), but you can still use your head and solid advice to make good choices, learn from experience, and end up with a worthwhile outcome.
(Oh yeah, and add asparagus. Always add asparagus.)