Setting good font-size for 980px mobile viewport (without meta[@name="viewport"])

Question

I have a very big website many different elements and generation of content. It does not seem practical to unconditionally add a meta[@name="viewport"] to the HTML of all the pages, because then it might break down various elements, and/or make some of the logos or graphics unnecessarily large.

We'd instead like to use a CSS media query like @media (width: 980px), (hover: none) {…}, and increase the font-size of actual text. This can partially be done automatically through -webkit-text-size-adjust, however, the feature seems 100% half-baked in that it simply accepts a percentage for the zoom without consideration of how big or small the viewport is, which would vary even on a single device w/ landscape vs. portrait mode.

• Question: Is there a way to determine the appropriate font-size that I should use in my 980px viewport to match the default font-size that would have been set had viewport's width been set to device-width?