Where do the measurements start on the extensions? Do they start at 12 inches? How far over can the stop flags move?

The scales begin at 11-3/4". With the extension collapsed, the scale ends at 58-1/4" which would be the further point for the flag stop. There's an additional stop on the end of the extension that slides out as well that can be used for repeatable cuts up to the maximum length of the extensions, which is 94-1/2". We will add photos to the product page.