You're not alone in your viewpoint, Rich.
I did a little 9 part analysis back in the '90s concerning recovery failures and attempting to find a more rational approach to understanding those failures from an engineering standpoint (I'm an Architect, so this stands to reason). Building structures are not designed on the premise, "Well, Joe Blow over there used a 12" beam in his building, so it must be good enough." (???) If one is going to design a structural member, you need two things 1) the nature and magnitude of the loading and 2) the energy resistant properties of the structural member that must *resist* that loading.
Recovery is an incredibly dynamic and complicated time in the flight of a rocket -- and hard to quantify -- thus 'rules of thumb' apply everywhere (not always the best approach). In fact, I have a rather low opinion of 'rules of thumb' - they're really only good a measuring thumbs (and then only 50% of the time). Surely there's something better.
There is -- it's called the 'Modulus of Rupture' (M(r)) (which gives one the energy of rupture of a material - expressed in Joules). Another item to look at is the 'Modulus of Elasticity' (a smaller number than the Rupture modulus, but gives an idea of how a material will react to loading (up to the Elastic Limit)) -- and is expressed in 'in/in' -- which gives you some insight into how 'long' a linear tensile member should be (like a shock cord). A longer cord can handle more energy than a shorter (in addition to it's other benefits) - and this is because, in textile materials, that total energy (Modulus of Rupture) is *mass* related. Cord 'Y' that is twice as long as cord 'X' will weigh twice as much and can absorb twice the energy, as well. Being longer, it also will have unit elongations that are half (that in/in thing) - thus lowering the stress (and dynamic shock) the system must take.
The bottom line in a lot of that research resulted in a number -- 76mN/TEX for the M(r) of nylon. Another way of expressing that unit is 'J/gm' - which simplifies the analysis greatly - just weigh your (nylon) shock cord and you can calculate how many Joules of energy it will take to rupture. It is equally simple to calculate the energy (in Joules) of a particular rocket traveling at a particular velocity. Compare the numbers and make sure the shock cord is the larger of the two.
Now, that really only gives us the ultimate strength of the shock cord - not how long it must be (well, assuming the mass of the cord is enough). I've always felt that the tensile rating of the cord should be between 50 and 100 times the dead load of the vehicle. Some may say that is extreme, but I've got some examples of dynamic (shock) loading multiples of 82x the dead load, so I think my numbers have merit. I'm also of the opinion that worrying too much about that nose cone 'hitting the end' is a little neurotic -- what is going to happen to the body tube when that cord hits the end? Sit there? (in contrast to being accelerated by that force input). Don't we remember Newton's Laws? If everything is of sufficient strength, whatever force is input, it will handle it (but I agree that too short is too foolish).
As far as 'rules of thumb' (as much as I don't like it), I would say, select a cord with a tensile strength somewhere between 50 and 100 times the weight of the rocket -- and (this simply recent thinking) use 5' as a 'unit length' (i.e. minimum) - then multiply that number by the *diameter* of the rocket in inches -- i.e. 1" rocket = 5ft -- 4" rocket = 20 ft. At least diameter is a closer number to being indicative of of the mass to be recovered than the length would be (again, factor in a healthy dose of skepticism for *anything* called a 'rule of thumb'. Do the analysis -- make a rational choice).
I'll close with a quote from one of my sources (from that research) that was quite relevant:
"The work of rupture (Modulus of Rupture -jhc), which is the energy needed to break a fibre, gives
a measure of the ability of the material to withstand sudden shocks of
given energy. When a mass 'm', attached to a textile specimen, is
dropped from a height 'h', it acquires a kinetic energy, equal to 'mgh',
and, if this energy is greater that the work of rupture, breakage will
occur, whereas if it is less, the specimen will withstand the shock.
Thus, the work of rupture is the appropriate quantity to consider in
such events as the opening of a parachute, a falling climber being
stopped by a rope, and all the occasions when sudden shocks are liable
to cause breakage. It should be noted that the significant feature in
the application of the work of rupture is that the shock contains a
given amount of energy; the fact that is occurs rapidly is not directly
relevant, though the rate of loading will affect the value of the work
of rupture."
"Physical Properties of Textile Fibres"
- W. E. Morton - p.271
-- john.
