I usually take the 3 measurements and then get an average. However this doesn't really work when doing mitered corners which I did on my last quilt a few weeks ago. I laid it out after finishing the top and realized that a couple of corners looked puckery so I took another seam on the mitered corner to straighten it out. My quilter called me today to tell me that, even though it quilted fine, that she figured out that the borders were actually a little tight for the quilt, which means that I probably shouldn't have taken the extra little tuck in the miter. Oh well, live and learn.