I'm not sure I understand this either. Are you trying to get the index polygon to fit perfectly within the data frame extent? If that is the case then you need to make sure the data frame size / aspect ratio is identical to the size of the index polygon. You also need to make sure that "best fit margin" is set to 100%.
For example, if the extent rectangle = 1000 meters x 500 meters then the aspect ratio is 2:1 and therefore the dataframe size needs to be 2:1. For example, 4"x2" or 8"x4", etc.
If I'm not understanding this correctly, please provide a screenshot with annotations labeling the problem.
Thanks,
Jeff