Possible variables to factor in would seem to be:
1) Mass of rocket
2) Mass of nose cone
3) Motor class (=> ejection charge strength)
4) Size of BT (practical consideration)
The other major factor is the cord length and the elasticity of the cord.
When the ejection charge goes off that energy forces the cone/body pair to fly apart.
Then aerodynamic drag slows the two parts down with respect to each other.
If you had a long enough cord (with sufficient altitude) then the cord would not need to be very strong - just enough to support the weight of the pieces.
However, we typically cannot put in such a long cord.
So now the cord needs to absorb some of the energy from the ejection charge.
The shorter the cord, the more energy. More energy, the stronger the cord needs to be.
In addition, a kevlar cord has virtually no elasticity so when you get to the end of the cord you'll have a sudden shock as the two pieces need to bring their relative velocity to 0 "instantly". Hence you need a longer cord to allow the aero drag to slow the pieces down some first.
Whereas an elastic cord will stretch (to its point of breakage) to gradually absorb the energy of the two pieces flying apart. And then pull the pieces back together again (someone say Estes "smile"?). Again, a longer cord will allow both the aero drag to slow the pieces down before needing to stretch and to slow back down from the elastic pulling the pieces back together. You can trade off a longer 1/8" elastic vs a shorter 1/4" elastic in terms of strength vs space.
Then you also need to factor in strength of the material vs bulk, type of elastic (straight rubber, braided, knit), safety factors (both to include special cases like a super shotgun ejection charge and wear/degradation over time), etc.
Bottom line - I don't think there is a simple "For situation X, use Y length of material Z" solution