When we aren't sure how to make something, we do mock-ups. Usually we make them out of scrap fabric, but here is a mock-up out of the actual product, a sort of balloon-London hybrid.
Originally we mocked this up to see about trim placement, but in the process we also worked out a way to shorten up the tails.
The client does not like the tails to be longer than the shades. Tail length is just a numbers thing, it's a product of the size of the return, the size of the side section, and the depth of the pleat. In this case that added up to quite a long tail, as you can see on the left side.
We pinned this shade together and attached the rings, then experimented by pinning a sharp taper on the right side to get the tail length we wanted.
Decorator and client are happy with the results, so now to finish this one and make two more!
If I don't forget, I'll take pictures of the layout as we make them, and the final shades.