I don't know about formulas, I work off scantlings and weight budgets. If someone else is doing it it has to weigh a reasonable amount, and it has to come from about one of 3 wall sizes if it is/could be 4130. I don't bother with calculations, though it would certainly be reassuring if I could. I know enough seat of the pants engineering to know what happens in gross terms when I mess with tubing diameters or wall thicknesses, but I can't "run the numbers". If I could I would still need some fairly difficult to arrive at info for the structure and the loads it is going to be subjects to.

Two other things are how the stuff works, and how much of an extension one is working with. Is it a seat post less design, or say something like a Bike Friday. And what does it interface with from basically nothing in the former example to ferules on the low and and seat posts on the upper in the BK case. These issues can affect sizing also, and often allow one to reverse engineer/copy a part. My guess for the former would be .035, and the latter something as heavy as 0.58, depending on whether that would feel like a crowbar or not. If it felt like half my weight budget, I would drop down to .049.