Converting a fixed pixel design to a fluid proportional layout

Graphic composites made in a program like Photoshop, Illustrator, Fireworks (RIP), or Sketch all have fixed pixel dimensions. At some point, the designs need to be converted to proportional dimensions by a developer when recreating the design as a fluid layout in a browser.

There is a beautifully simple formula for making this conversion that the father of responsive web design, Ethan Marcotte, set down in his 2009 article, Fluid Grids (http://alistapart.com/article/FLUIDGRIDS):

target / context = result

If anything resembling math makes you quiver, think of it this way: divide the units of the thing you want, by the thing it lives in. Let's put that into practice as understanding it will enable you to convert any fixed dimension layouts into responsive/fluid equivalents.

Consider a very basic page layout intended for desktop. In an ideal world we would always be moving to a desktop layout from a smaller screen layout, but...