Gabel,
I agree with your solution, however we are often not able to go to the installation site before or during the installation.
We may be relying on an installer or owner that may not be well versed in shooting elevations. They may not even know how to use a spirit level. So...in order to have a good installation result, we send our pergola columns with a leveling device. I am interested in improving that device.
Thank you.