javascript evaluated width in a nowrap element

Question

I'm trying to develop a newsticker plugin, while i was making the sliding part i got an issue with calculated width, all seems to be correct but when i get into the strip where i should calculate the width to slide the inner content i get a width lower than what i expected.

The mask has right offset width, but i can be sure if inner content with nowrap parameter is correct, because when i substract mask width to inner content i get a lower value than the desired one.

The sample is here http://gist.github.com/351562

and the conflicting part is that :

var despWidth = offsetContentWidth - offsetContainerWidth;
console.log((despWidth*-1)+'px');

Anyone knows how to get the right inner content width?