The tracking set on the Format tab of a field is different that the charspacing that is specified in a VB or HTML field script using <stylerun charspacing="x">. Tracking on the format tab is designed to imitate Quark Xpress. This formula uses the standard kerning value, as defined in the Quark help file. Quark takes the font size and calls that an em space. The kerning is 1/200 of the em space. So to convert this to a value the system uses this formula: (fontsize/200)*kerning.
If you are using Adobe InDesign, you should bear in mind that ID measures tracking in 1/1000 em, not 1/200 em like Quark. Therefore the formula would be (fontsize/1000)*tracking if you are taking the value from InDesign.
The charspacing in a VB or HTML script doesn't do this calculation. The value you enter is not applied to the font size; it is an absolute value. Therefore you will see a bigger difference when applied to a smaller font size. We do not have an exact computation you can use here; all we can say is that larger positive values will stretch the text out more and larger negative values will tighten the text up more. See this example: