I'm glad you liked the article.
Answering to your question, the value of "40" comes from a set of pixels arranged in a horizaontal or vertical line at the time of VGA screen resolution.
The maxium capacity of vertical arrangement in mobile VGA screen was 640 and in a horizontal arrangement it was 480.
But to maintain the 16:9 aspect ratio (landscape-widescreen) in a device which was supposed to be hand held, designers and engineers kept the VGA resolution to 360x640, and if you'll calculate 640 to it's aspect ratio counterpart 16, you'll get the value "40", which was used to calculate "360" by multiplying 9 with 40.
You can read my same Artboard guide but for desktop on my profile to learn more about VGA and XVGA screens.
Hope that answers your question.