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.
Yes, I mentioned that the formulae are similar, but there is no threshold that I am aware of where the rules change after a certain angle. As you pointed out, the force pressing the surfaces together is what creates the friction. That force can come from many sources. In the taper, the force is not applied perpendicular to the surface but rather, as you correctly point out, thru an angle. If you consider each surface as non moveable, then the calculation is not complicated. But all materials are elastic. It is only a question of degree.
Bolts are a lot easier for people to comprehend. While the calculations are again straightforward, they are not directly concerned with the amount the bolt stretches, but rather the tension generated. The stretch is definitely there in any case. Take the practical example of putting 100ft lbs of torque on an ungraded 'hardware store' 3/8" bolt. It is unlikely the bolt will ever reach the 100lb spec. It would just stretch to failure. A grade 8 bolt would generate an extreme amount of tension. The difference is in the material. If manuf #1 us using bolts from Home Depot, he may well have a different spec than the manuf who use using graded fasteners.
My intention was to point out some of the non-so-obvious, but still very much present factors that might provide some insight into why one manuf would have different specs than another manuf for the same CAT40 taper and why both might be correct in their application. I think it would be safe to say that my X3 spindle would never survive a 4k or 5k pull on a taper. Considering the material that spindle is made from, we would call that a swaging operation . . . LOL!
I think we all do the same thing, Ray. 1) We run the numbers and then 2) build a prototype and then 3) try the prototype and see if the calculator lied or not. Some folks skip step 1.