Question about nucleus.css grid and 2/3 - 1/3 content/sidebar layout

I notice that the grid system in nucleus.css supports single column sizes (1/2, 1/3, 1/4, etc..). PURE css also supports columns but also includes things like 2/3, 3/4, and other variable column sizes. I also read that PURE css is just a fallback in case a browser doesn't support flex, so I'm assuming that the PURE classes should not be used.

If I wanted a 2/3 content area with a 1/3 right sidebar using nucleus without using the multi-column width css from PURE, should I incorporate the .block class? This seemed to work but I'm not sure if I'm using it right.

   <div class="grid">
      <div class="block">
       content
      </div>
      <div class="size-1-3">
         sidebar
      </div>
   </div>
---

You're close, you just need block on both to trigger the flex box columns, something like:

<div class="grid">
  <div class="block">content</div>
  <div class="block size-1-3">sidebar</div>
</div>