Need To Build A 'Progress Bar' With Javascript

I've got the following problem: i need to build a "progress bar" with
JavaScript. So far i have the following configuration: i've got a DIV
inside of a DIV. Each DIV has a style attribute that defines its width and
other style information.

I need to calculate the size of the incremental block based on various
factor like Width of the outside DIV, incremental interval and number of
increments. My original solution was to assume that all of the units are in
'px' so i could just deal with integers. However now i hit a brick wall,
when i realised that this is anything but scalable.

So what i need to do now is to obtain a width of the DIV from its style
attribute (had it hard coded before), do the calculation, and then use the
result to increment the size of the inner DIV (width element in the style
attribute)...