Rff1010, If the quilt is 78 x 88, for the longarm I would want the back to measure at least 86x96 (4" all around). 96" is 2 2/3 yards. Two lengths would be 5 1/3 yards, and wouldn't be wide enough for your 86" width. Check your quilting calculator again, because 5 1/4 yards would not be enough.
If you cut your 5 yards in half, you will have two 90" pieces. However you could cut it into two 86" lengths and have 8" left over for piecing. Assuming that you can get 40 usable inches in WOF after seam allowances, you would then need to make a stripe that finishes at 86" x 16" and insert it into the two 86" lengths you've cut from the 5 yards.
On the longarm it is best to have the backing seams running across the bars, rather than down. So if your quilt needs to be longarmed with the 88" of the top lying across the bars, you should consider a different piecing strategy for the backing.
I hope someone checks my math.
