I have read the book "Responsive Web Design - By Ethan Marcotte". There I ahve learnt the forumla "target / context = result"

Those who have read the books, you saw that the main container at first got a width of "960px". But after converting it in percentage, Ethan set that as "90%". But actually, 90% is not equivalent to 960px.

Let us take out the following example:

My screen resolution is 1280x1024. Usually, my browser width in full screen is 1280px. Suppose that I want to design in 960px. So if I want to make it percentage, it would be
960 / 1280 = .75
And if we take the decimal after two numbers, it's 75%. But 75% and 90% is not the same. So if I want to design a fluid grid which is actually equivalent to 960px, what should I do? So to say, Ethan did the same thing in his book, set the main wrapper width 960px and later to 90%. Practically, when I set one of my 960px design in 90%, there was a vast difference; at that time it didn't followed the grid system of 960 (I also tested a grid overlay with Jquery plugin).

Okay, now I suppose that my width is 90%, but what should be the "context" in the target / context formula? - as I need to mention the context in px's to get the rational percentage. It's surely not 960px.

Any suggestions?