I've been reading up on this and have found that the longer is better... but I'm 5'5", so... I don't need a 10 ft hammock. I've read that height + 10"-16" for whipping + 24" (one foot extra on each end)= should be plenty. Lori's suggestion is good too, since you will be able to customize it for your body a little easier. Personally, there's no way I'm going to start with 4 yards of fabric. I just don't need that long of a hammock.
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I always forget and make it more complicated than it needs to be...it's just walking.