template responsiveness issue

With the help of this forum (thank you, paulwpxp), using the TwentyTwelve theme, I created one template (called text-page, derived from full-width) that would enable my text-heavy pages (no sidebar) to be narrower than my other pages — 600px vs. 960px for my other templates. The overall container remains 960px wide, but the text within it is 600px wide, which I find more readable.

Now I’ve discovered that when narrowing the viewport on my browser, to test the new template’s responsiveness, the 600px text width breaks down. The wide margins disappear and the text column jumps out to 750 or 800px wide or so. Sorta sloppy.

Can anyone advise how to adjust the responsiveness of the template to “break down” nearer to (or at) the 600px width?