I think we should not even generalize it down to a rule of three, because then you're outsourcing your critical thinking to a rule rather than doing the thinking yourself.

Instead, I tend to ask: if I change this code here, will I always also need to change it over there?

Copy-paste is good as long as I'm just repeating patterns. A for loop is a pattern. I use for loops in many places. That doesn't mean I need to somehow abstract out for loops because I'm repeating myself.

But if I have logic that says that button_b.x = button_a.x + button_a.w + padding, then I should make sure that I only write that information down once, so that it stays consistent throughout the program.

The reason for the rule of thumb is because you don't know whether you will need to change this code here when you change it there until you've written several instances of the pattern. Oftentimes different generalizations become appropriate for N=1, N=2, N>=3 && N <= 10, N>=10 && N<=100, and N>=100.

Your example is a pretty good one. In most practical applications, you do not want to be setting button x coordinates manually. You want to use a layout manager, like CSS Flexbox or Jetpack Compose's Row or Java Swing's FlowLayout, which takes in a padding and a direction for a collection of elements and automatically figures out where they should be placed. But if you only have one button, this is overkill. If you only have two buttons, this is overkill. If you have 3 buttons, you should start to realize this is the pattern and reach for the right abstraction. If you get to 10 buttons, you'll realize that you need to arrange them in 2D as well and handle how they grow & shrink as you resize the window, and there's a good chance you need a more powerful abstraction.

> Instead, I tend to ask: if I change this code here, will I always also need to change it over there?

IMO, this is the exact (and arguably only) question to ask.

Critical thinkers understand that rules aren't written for critical thinkers; that they are written for beginners who don't yet have the necessary experience to be able to think critically.