There are several tactics for deciding where to put breakpoints in a responsive design. There is the rusty idea that they should be based on common screen sizes, but this doesn’t scale well. There are no “common” screen sizes. Another popular tactic is to create a breakpoint wherever the layout breaks.
This sounds much better. But it still leaves us with the question, How do you determine whether the layout is broken? One logical answer is to look at classic readability theory and to define our breakpoints based on that.