I found a video that I thought was interesting. It's in French, and it features the Red Blue Chair, and other Rietveld designs. If you want to see the Red Blue being built at the Cassina factory, go to the 9:20 mark.
Click here to watch on YouTube
If you want to see exactly how I got the dimensions when I built my Red Blue Chair, over 20 years ago, watch the video at the 18:00 mark. It shows a man in a museum sneaking around the chair measuring, and writing the dimensions down. I did this same thing (there was no Internet back then to find plans) but instead of a museum I went to a modern design store in San Francisco and took some measurements when nobody was around!