I am new to this so please indulge me.On the DT BB mitre there is the extension that represents the seat tube. How does one use this mitre? I assumed that small intrusion of the seat needs to be cut out of the downtube. I also assume the vaguely diamond shape part of the miter template that sticks out one end is not cut out.
There is a description of how the BB DT mitre template is meant to be interpreted at the bottom of the page at: bikecad.ca/miter_templates.
Let me know if that does not answer your question.