Whoops, I see there was more to your post. Nevermind.
No problem.
The calculations are necessarily somewhat loose, and dividing figures of two significant digits does not give fine-grained results. :) We'd have to have the raw data from those thousands of phones, to really do it right.
My point in the bit of arithmetic was only that the original article was hyperventilating about the 82% front-plus-back figure and implying that it was -all- due to materials and mechanical design, and ignoring the back as a new breakable item.
It's still the case that Apple should be looking at the mechanical arrangement of things in that phone. I have a bet that the iPhone5 is significantly different.