I read the article you referenced athttp://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/wat...

It doesn't say WHERE the 41,045 BTU number cames from.

What is this "magic" number of 41,045 BTU?

Wrong reference.
http://www.waterheaterrescue.com/ima...yAndNight3.jpg

The right reference is
http://www.gamanet.org/gama/inforeso...s?OpenDocument

The document that has that calculation is
http://www.gamanet.org/gama/inforesources.nsf/vAttachmentLaunch/C2AAFB8D41D003F485256E9000607F66/$FILE/12-07-gas-rwh.pdf

But it still doesn't say where the "magic" 41,045 BTU comes from.