Set Tracking value

Hi,

Need alter for tracking value.

My last line character count is less than 10 in the paragraph, I want give -10 tracking value in the particular paragraph

My last line character count is more than 70 in the paragraph, I want to give +10 tracking value, in the particular paragraph

This is my script for tracking pls help

var myDoc=app.activeDocument;

var myTrack=app.selection[0].paragraphs.item (0).tracking=4;

pls correct it

by hasvi