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Sailboat Rig Dimensions for boat starting with B
you will find your rig dimensions (scroll down to your boat) - the Spinnaker luff dimensions should equal your "I" measurement, the max girth (width) should be about 1.8 x your "J" measurement to rate as a standard kite.
Finding a used chute shouldn't be difficult, and approximate dimensions will be fine esp. if you're not racing or worried about handicaps. (maybe go slightly smaller rather than slightly bigger unless you're in a really light-air region)