I used to do custom draperies... The normal rule of thumb is to have the cornice 1/3 of the height of the window. Take a piece of paper cut to that length and have someone hold it up to see if it is enough - you can always make it longer if you are taking it up to the ceiling. It looks silly to have a skinny valance. I would love to be there to help you. I worked in Valos Verdes and did lots of odd treatments - we even used to do the draperies that were curved at the top - a bugger I might add, but once you learn the tricks, can be done...