Just how good is the 20/1.7?

Pekka Potka has claimed that the 20 is actually more like 18.5 mm and I believed him. However, after being prompted to do so in another thread, I took the trouble to measure it using the method described here:

http://www.panohelp.com/lensfov.html

My result: The 20 is a 20, no more, and no less.