OK, math was never my thing. I have tried to do the 'ciphering' and my head is all jumbled up. This is only the third quilt that i have made and for the last two, I just had the long arm lady use her muslin for the backing, but on this one I found a companion fabric for $3.98 per yard at Hancock's of Paducah so I figured I can actually afford the extra money and have a matching back. My quilt is 92 x 80. How much do I need to buy for the backing? (the long arm lady likes to have some extra to work with also)