Steve,
Is that really correct? I would think tapers are purely a matter of friction. If you know the material and surface finish of the inner and outer tapers, you know (or can measure) the coefficient of static friction (assuming a completely clean, dry fit, as it SHOULD always be). Knowing the drawbar tension and taper angle, you can easily calculate the normal force. Multiply the two, and you know the total frictional force. Integrate that over the entire taper, and you can calculate the total torque that can be generated before the taper slips. I can see how stretch might be factor on low-angle tapers like Jacobs, but have a hard time believing it's a factor on CAT/ISO tapers, especially given the relatively low drawbar tensions, relative to the cross-sectional areas of the toolholder tapers.
Regards,
Ray L.