Nothing against Carl's response, this has been a general rule of thumb for as long as I can remember, but I've always felt that the "rules" for efficiently and sufficiently installing shock cord are way too general!
For example, I've used kevlar only in an astromcam test subject and even though I only used an Estes C11 with its associated BP ejection charge AND over 6 ft of kevlar, I still had the weighted (exactly the astrocam film-loaded weight) nosecone break (on ejection) a decent quality swivel right in half...thus losing the nosecone. So this cord was about 3 times the body length...
Then I've had BT80 size rockets (say the Estes Optima/Shadow) with relatively short (2.5-3 feet) 1/4 inch nylon elastic only survive many many flights fine (except the core sample that was my failed attempt at a new improved and cheap baffle!!
I've had more success with kevlar/elastic nylon combos in shorter smaller diameter rockets (I think the bungee effect saves from having to use a mile of kevlar to avoid the astro test above) and kevlar avoids the cord cooking inside...
and for large LPR/small MPR (24mm reloads and SU motors, Estes/LOC parts) just using 1/4 inch elastic 4-6 feet or so is fine... also can depend on how much stuffer tube on BT80 type rockets... like old Shadow had only 10 inches or so of parachute tube with the rest of the 4 foot rocket having a 24mm stuffer tube. Then theres TLP kits that don't use stuffer tube but are equally as long... so BP from a D12 say can't push nose out as hard...
I guess what I'm trying to say is that there is no "unified theory" on this that really works in my experience and a few model types should be referenced for much better guesstimates..
i.e.
small diameter non-superroc types = kevlar tied to 1/8 inch nylon
BT80 or so with/without stuffer tube
anything with extra heavy nosecone gets special treatment...
etc...
The problem seems to be not so much length related but internal volume to pressurize for the designed motors and the weight of nose and/or recovery system....
what do you all think? How to better categorize rockets for this? Just my thoughts.....