I understand most of what you are saying Peterpan. I am not a numbers guy or engineer either, I have learned most everything by trial and error but I also like to hear from different people and weigh the lot of it, but in the end you are back to the actual test. You lost me a bit with the numbers and what is BK case? All I can tell you is that I am planning on using an externally butted seat tube and am building an aluminum seat mast/seat cap similar to the Ritchey stubby seatpost. I don't want the frame to ride like a jackhammer but its better than a failing seat tube. This is my first design of this product and I have to rely on my customers for feedback and I can alter it accordingly. The reason I asked about formulas is that a bike mechanic told me he had one for determining stress under a similar circumstance but couldn't remember it at the moment. There are probably so many variables that playing with numbers is hardly worth trusting, back to trial and error.