To answer your question requires much more information from you and many long detailed answers from me. I would literally have to write a book to explain it ...what a minute...that is what I have done :-)
The best answer I can suggest is to buy my book...not so I can get another book sale, but because it will explain to you how the six block models work and how they are affected by all the CSS properties and property values like percentages. Lastly, it will take you step-by-step through the process of creating fluid layouts.
My fluid layout design pattern works. Just check out the examples in my book. They work in all major browsers at all screen resolutions and they even work in PDAs and cell phones.
I understand your doubts because before I invented this breakthrough design pattern, and I never could get floated divs to behave consistently -- it all seemed like a black art of trial and error -- mostly error!
For example, I just now pulled up my website,
http://cssdesignpatterns.com on my blackberry. The entire website is based on the fluid layout design pattern and the pages look great on the blackberry's tiny display. I then navigated to the Fluid Layout examples in Chapter 17 and they displayed perfectly--wrapping around to fit the display on my blackberry. Even when I turn off CSS in my blackberry, these pages still look great. This is the true power of my fluid layout design pattern. It just works!
Unfortunately it would take about a dozen pages of text here to explain how and why this design pattern works so well, but you can find it all in my book :-)