Illustrator Scripting :: How To Export JPG

Quite new to Ai's scripting stuff.
 
I'm having a complicate time with Ai's save to jpg options vs artboard sizes and units conversions.
 
Say, i have a doc (from my client), which they inherited and enhanced for 10 years may be.
 
It's "supposed" to be in millimeters, but artboardRect gives me those values :

-0.27880000000005
436.191388980878
646.457
-0.20900000000074
 
which sound like weird points or pixels.
 
I need to export a jpg (or gif, or png), that will be X pixels wide. For the purpose of the test i set it at 1200.
 
var maxLargeurPx = 1200;
 
First thing, getting the artboard's width :
 
largeurArtboard = Math.abs(thisArtboardRect[2]-thisArtboardRect[0]);
// 646.7358
 
Then i try to have a multiply factor to use
 
coefExport = /*Math.round*/ (100 * maxLargeurPx / largeurArtboard);
// 185.54717397738
 
and then set my ExportFile jpg to this value (i can't give it a number)
 
     // exportOptions.horizontalScale = coefExport
 
That's when it's getting fun : my images are 1204 px wide with or without clipping.
 
Of course, i "can't" resize the doc for each export, especially expecting to have dozens of those, with different values, dimensions, etc.