My best advice is to do one miter at a time. E.g if you have to set 2 inches left and 3 inches up, miter your 2 inches left and then do your 3 inches up off of the already mitered duct. I haven't done it in the field yet. There is a formula to do it in one measurement and I am looking for the answer. Will get back to you . Let me know how it goes if you try it the way above
