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Gram, whether you are making this back to be quilted by a longarm quilter or you on your DSM, you do need 1/2" seams in the backing - for some reason they lay flatter then a 1/4" seam...and don't use the selvedge in the seam as it has no stretch and the rest of the fabric does. As Judi said, make the back asymmetrical - much more interesting and so much easier!!!
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Sounds like you are quilting it on your DSM? Suggest you glue base, sketch ahead of time, etc. Very unlikely that your LA quilter can get it perfectly lined up--think about rolling to towels together--at the end they don't come out the same--same principal with long arm and part of the reason that you need extra backing.
Either way, be sure that your back pieces are on the straight of grain-especially if you are using larger pieces, so that you don't get stretching that results in tucks,etc.
I would allow some leeway in your design for the backing.
It is difficult to line up a back "exactly" with the front.
Remember it is the back....the front is the focal point.......don't agonize......how often do you look at the back of a painting? ....or the quilt on your bed?
When I piece backs to quilt on my DSM, I line it up with pins while basting. I'll put a row of safety pins horizontally and one vertically through the center of the back and line them up by "feel" to my front design. It's worked out pretty well for me that way. Remember to take the pins put of the back before quilting!