(89 days until the 2012 WHPSC)
I read somewhere that mitered tubes could benefit from reinforcement plates. I think the purpose of the plates is to maintain the elliptical cross section, preventing distortion.
The first welds (yes, plural, as there are two root passes followed by a larger pass).
A quick check of the angle tells me I'm right where I want to be.
Getting ready for the second set of welds:
Here's a closeup of the reinforcement plate before welding: