Here's what I did (after it was suggested on these forums sometime last year):
--Pinned up a long sheet of stiff paper on a wall (builder's kraft paper works well).
--Hung a length of 1/8" beaded chain corresponding to the length of the seam over the paper. Adjusted the tension of the chain until the deflection looked good. My longest seam was (IIRC) a little over 6' (roughly 200cm?), and deflection was roughly 1.5". Less deflection for shorter segments, obviously, but if you keep the tension more or less equal, that will happen automatically (ie, shorter span, identical tension=less deflection).
--Sprayed a line of spray paint over the chain, marking the paper with the chain's "shadow." Because beaded chain has little straight segments between the round balls, the line was quite accurate.
--Took down the paper, cut out the curve, used as template to cut fabric.
I like this method better than spreadsheet, CAD, etc, as I still would have had to plot out the curve on paper and cut out. I have access to a plotter, but printing costs were more than using the chain method. And plotting points by hand seemed like a much longer process than using the chain.