I'll confess that I copied, a lot. But I also took the time to check and do the calculations to make sure that I understood what I was doing, not just imitating.
The calculations are nothing beyond 1st year mechanical engineering. You can go nuts and do lots more computations, but when starting with something that already does work, it should be no surprise that the math tells you it works.
If you didn't happen to take 1st year mech eng courses (everyone in the world should, really) but want to know everything about it anyway, then this turns into a lecture about free-body diagrams and strength of materials, and stops being about towers for a very long time. The internet just isn't a very good medium for teaching that kind of detailed stuff. I've written a few things about it on my personal web site, you've found some good work by Cardamom already, and the Bergy tower install manuals are priceless.
You aren't wrong: the Bergy towers can be over-designed, but see it from their perspective: they are selling a product for installation in locations god-knows-where. The last thing they need is a tower dropping down because they assumed a safety factor of 4 was good, but somebody overloaded it by 5x. Which is also a lesson for us DIY builders to heed as well.
I think that if you keep looking you will find more information about the subject, but little in the way of detailed "how-to". The cost of making such a "how-to" is very high, so you can't expect it for free.
