"Mobile first" is recommended, I know. However, I understand this to be because of the progressive enhancement principle. But when I build a simple site, where I am just scaling/re-arranging items to fit better and have better legibility on smaller devices, can I not just as well start with the desktop design and work my way down to mobile?
Best How To :
It's near impossible to design something perfectly for every possible screen. The basic "breakpoint" as you put it has always been to determine what the client needs, and what they expect. Presently, most clients need a site that meets some basic mobile requirements or at least has a few mobile-friendly pages, even if deeper content is still in a desktop format. There is no one fixed answer for this. However, you should probably be conceptualizing how your design will work in both formats and trying to minimize the amount of rewriting you'll need to do from one to the other, by keeping the layouts as fluid as possible.