Hi Rombout,
I know it is more then two months later, but maybe you find this interesting for another time.
The image you posted is in ‘one point perspective’. The verticals are perfect vertical, the horizontal lines are perfect horizontal.
I give an example made with LibreCAD.
You can find vanishing lines that go the center (cyan). Now we have the vanishing point. This point is in ‘one point perspective’ also the center of the image. Then you know the horizon lines (yellow).
We want to find the ‘diagonal vanishing points’ of the image. The chair has two own vanishing points (blue lines) that are on the horizon. Now we make a (blue) circle between the chairs vanishing points. The blue circle intersects with the vertical yellow vanishing line. Because of Thales’s Theorem, we found the diagonal vanishing lines that give a 90° circle of view (green). This circle cuts the horizon. we found the diagonal vanishing points in the image:
With this information we can setup the framework that the camera calibration add-on can use. Here I give the rectangles:
Use here Solve Focal+Y, because the verticals are vertical.
Example.
I hope this information is useful.