You cannot escape environmental factors. As you cook more and more, you will understand your kitchen and environment more, and learn by feel what "medium heat"

You cannot escape environmental factors. As you cook more and more, you will understand your kitchen and environment more, and learn by feel what "medium heat" is, and how it affects food, and why you might use it. These things are always guidelines, and intentionally vague. The most famous example is how most baking recipes call for 2-5 cups of flour, since the humidity in the air, the season the flour was harvested, etc. all affect how much you might need in the end. So you just learn when you've added "enough". That might make the engineer in you upset, but that's the reality of it. There are no precise steps to "brown an onion", but you will know when you've done it correctly. And if you did it wrong, that just means it takes more practice and more learning about your skills.