Targeting responsive styles with child theme

Hi, I’m trying to make a child theme for Flat, and need to make some adjustments for the mobile CSS. It looks great full sized and when the width is less than 680px or so, but it goes through an awkward stage around 1000px width.

At what width exactly does the page start to reformat? Is there a way to make it stay in the normal layout until the window is about 600 or 800 px wide? I’ve done this through media queries on other themes, but it has been harder to figure out what to target in this one.