I would get an extra 1/2 yard, since you will need to make the backing wider than 108" for your longarmer.
Personally, I like splitting my fabric 1/4 and 3/4 and then adding in a second color as a stripe. You could split your 108" fabric at 27" and then seam in 18" of a second fabric (1/2 yard by 118" or whatever your longarmer requires), then add the additional 81" of the wide back fabric. Trim as needed. And I press my seams open on the back since they don't nest.
If you are really trying to save every bit of fabric you can & are willing to have one area of your quilt that is heavily pieced, you can order 130".
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