If I have a semi-circle window 55� long and a radius of 24�...how do I order enough to cover that, in running feet?

Take the total length and total height. These would be the measurements. The curve of the window would have to be trimmed during installation. For more information on measuring for round windows, click here: https://www.decorativefilm.com/mm5/graphics/00000001/How%20To%20Measure%20-%20MTSF-1.png