If I were you I'd experiment with a string. For one individual pane, I'd attach it at both ends and test the look of the drop until I was satisfied. I'd take down the string, measure it, and use that measurement as my length for the individual panel. Allow enough for the casing that holds the curtain rod.

I refer to this as a "Canopy". I would purchase some inexpensive muslin and use it as a template for calculating the effect, amount of drop etc. Start by doubling the overall length first and just use push pins to simulate the effect you want to achieve. After you are there then take a picture and either have a professional fabric workroom sew it and install it. They will measure it and calculate it for you. Seems if you can sew a straight stitch, you could handle creating a similar look. Good luck with your project! This appears to be Burlap.

Ultimately I agree with Cynthia, experimenting with a string is a great idea. The length of your swag is going to be determined by many factors such as the height of your space, how you are hanging the fabric being used and of course personal taste. Using a string or thick ribbon to experiment with your ceiling swags is a great idea to insure proper measurement.
As an event decorator I have designed many ceilings with this technique and have always determined how much fabric to have on hand by taking the length of the room and multiplying it by 1.5. Once again always test out your measurements first. I use this calculations in ballrooms and tents using the same hardware each time. Something as small as how you hang the treatment and height of the room can change everything. :)

Thank you - I was looking for something like "multiplying it by 1.5" or "doubling the overall length," but couldn't find a calculation for this sort of thing. I hadn't thought about the height of the ceiling and how far the fabric would come down, so trying it out with string or ribbon is a good idea. I want to make it easy to remove for the winter - and for cleaning. We live in Colorado where there is a lot of dust!

You're very welcome. This is a very interesting and practical treatment and not difficult to execute if you take your time and measure twice and cut once! :)

In fact, why not buy some ultra cheap fabric, just three or four yards and do a mock-up before you do the whole thing? I think you could sew this very simply by folding the fabric, stitching in a line (in from the fold) half the width of the casing you need. If you do this, there's no cutting, no extra fabric tube you need to create and sew onto the fabric. You'll definitely have to allow for the fabric that this uses up, when you make your calculations.