How to calculate the ExportPNG24 scalefactor

I need to export a Illustrator icon in various different resolutions e.g. 16x16, 64x64, 256x256 etc.

I wrote a simple script for this purpose which is calculating the scalefactor out of the image width and the desired resolution.

The scale factor is calculated by this simple formular:

var bounds = item.visibleBounds;
var width = bounds[2] - bounds[0];
var scaleFactor = 100.0 * desiredPixelWidth / width;

The resulting images are correct for small images from 16px to 512px but for larger images the resulting images are too big:

(the calculated value for width is 512.5)

I have to multiply the scaleFactor with a correctionFactor of ~0.999 to get correct results:

var bounds = item.visibleBounds;
var width = bounds[2] - bounds[0];
var scaleFactor = 99.9 * desiredPixelWidth / width;

I am wondering if anyone knows why the correctionFactor has to be applied and what the correct value is.

Windows 7 64Bit / Adobe Premium CS5.5