In my opinion, there is really no "rule" when it comes to details. Yes, guidelines for proportion. Some jabots can be as short as the swag length for a simple cover to the return. Others can be very long, down to the sill. I find most tend to end up in the 1/3 to 2/3 length of the overall height. Will there be any stationary side panels or sheers? What might help is to search for images online and you'll see a huge assortment that will help you visualize the choices in your room.