Improving Precision when scaling to exact pixel size

Question

Hi All,
I'm very new to javascript and illustrator scripting, so I'm pretty sure my problem might be easily fixed
I'm trying to write a script for creating PNGs with a specified pixel size.  I have created the following functions, which first works out the scaling % to use based on the artboard size (borrowed the idea from this post -thanks MuppetMark), and then export the image to PNG.
It works, but, I am finding that when scaling to larger sizes, the resultant output is often a few pixels different from the value I have specified.  I'm sure this is due to precision/rounding issues in javascript, but have no idea how to fix.
I'd be really grateful of any suggestions
Many thanks
Tim
function getScaleFactor(artboard, width, height)
var scaleFactor;
if (width > 0)
        var scaleFactor = ( width / ( artboard.artboardRect[2] - artboard.artboardRect[0] ) ) * 100;
else if (height > 0)
        scaleFactor = ( height / ( artboard.artboardRect[1] - artboard.artboardRect[3] ) ) * 100;
else
    scaleFactor = 100.0;
return scaleFactor;
function exportFileToPNG24 (dest, scaleFactor) {
    if ( app.documents.length > 0 ) {
        var exportOptions = new ExportOptionsPNG24();
        var type = ExportType.PNG24;
        var fileSpec = new File(dest);
        exportOptions.antiAliasing = true;
        exportOptions.transparency = true;
        exportOptions.saveAsHTML = false;
        exportOptions.artBoardClipping = true;
        exportOptions.horizontalScale = scaleFactor;
        exportOptions.verticalScale = scaleFactor;
        app.activeDocument.exportFile( fileSpec, type, exportOptions );

Answer 1

What you type in that box is just the ratio not the actual pixel number. And the box does not like to many digits in there. A ratio of 4 x 5 would work perfectly fine in your case, but more complicated ratios like 21 x 50 don't work.
To get 500 x 400 px you must then export your image with settings that restrict your image, eg, to fit into a 500 x 500 px box.
The idea behind this two-step approach is that you might think of 500 x 400 px output right now, but by doing the actual pixel generation at a later point, you can always easily export later at 1000 x 800 px if you need a higher resolution version with the same aspect ratio.