Measure your quilt at each end & in the middle; take the average of those 3 numbers and cut your binding to that length.
As far as hanging on the wall, the bottom will always be a tiny bit wavy. The only solutions I've seen for that are to sew in a weighted bar or to use a narrow sleeve & bottom mount (Caryl Bryer Fallert does that with her wall quilts to keep them from "waffling").
http://www.bryerpatch.com/faq/storage.htm#bottom