Reponsive Design: With only a Gazillion Viewport Sizes, what's a Girl to do?

I always make it a point to develop websites that are "mobile "friendly" which to me means it "functions and looks pretty good".  Recently though I've been working on creating websites with a "Responsive Design" where sizes and content 'magically' adjust themselves based on the height and width of the user's screen.

Here's a great resource that lists out the viewport dmensions of pretty much every device imaginable: http://viewportsizes.com/

Here are some of the most common ones:

iPad (1, 2, 3, Air, Mini):
Portrait Width: 768px
Landscape Width: 1024px

iPhone (3, 3G, 3GS, 4, 4S, 5, 5c, 5s):
Portrait Width: 320px
Landscape Width: 480px

Kindle 3, Kind Fire 2:
Portrait Width:600px
Landscape Width:963

Kindle HD 8.9
Portrait Width:800px
Landscape Width:1220