Its not that hard
You need to set the view option on your pc to "details" so you can see the files sizes of each file, the main movie will be the set of VOB files with the
largest size, that VOB set will have one VTS IFO file and thats the one you select with autogk
Your know if you do it right as autogk will display the correct movie lenght
Example
VTS_01_0.IFO
VTS_01_1.VOB
VTS_01_2.VOB
VTS_01_3.VOB
VTS_01_3.VOB
VTS_01_5.VOB
Have a look at the screen shot in the link, where I have put the Blue arrow is what you use to select "details" view
You can see the VOB set with the largest file size which is the typical size for the main movie
http://img301.imageshack.us/img301/4082/autogk2ft7.png
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