Welcome to the forum! I agree with John, bungie isn't the best for rigging the tramp. You want something stiff that won't stretch. The tighter you can get your trampoline, the more rigid the hulls will be (less flex) and the faster the boat will be. Some folks use bungie for their initial lace up, and then lace it up with the pre-stretched line to tighten it up. Just be careful that you don't pull too hard on any one grommet, you can pull them right out/tear the trampoline (though you may already know all that).
Hobie's part guide lists 19 feet of 1/4" line for the center lacing, and 2 pieces of 15 foot long 1/4" line for the rear lacing.
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