Ya know what scares me most about what you just typed? I think I actually understand it somewhat. Your explanation is put in a way that it makes some sense. I see now that you wrote it out, how the wall thinckness can be calculated. I still dont know, and probably thats a good thing, what the moment of inertia is to a tube. But I dont really have a lot of use for knowing much about that so........

 But one part I dont get on 8x12, is that it seems like the formula is for the rectangle tube laying flat instead of 12" tall and 8" wide. ? Is that right?

 Thanks a bunch for that, and taking the time to explain it to me. That really does help a lot.