Russell's fascination with pyramids comes from some Brits who felt that the pyramid inch at 1/25th of a cubit and very close to the British Imperial inch was of divine origin and superior to the "metre" invented by those pesky French. (There were also wacky ideas that proto-Brits had built the pyramids and that the Brits were the lost tribe of Israel.)
Russell liked the idea of the divinely dimensioned pyramid and gave a lot of consideration to the idea that the dimensions of the internal structures of the Great Pyramid foretold history including the return of Christ. He arrived at 1914 based on these measurements although he never set foot in nor placed a tape measure to the Great Pyramid of Giza.
On top of these wonderfully loopy ideas on Pyramids Russell also used a set of numerical gymnastics. Basically, 7 times (from Daniel) that weren't times but actually years but not really years but years of days. These aren't really days but days means years based on 360 days to a year. (Farkel does a much better job of explaining it.)
Thus, using pyramids and Biblical numeracy Russell calculated a 2520 year period starting from 606 BC. He forgot, or didn't even know, that our dating system was invented before we were taught Math by the Arabs (a benefit of the Crusades) and as result there isn't a year zero. So using the wrong date for the fall of Jerusalem (606 is neither accepted by historians or JWs) Russell arrived at 1914. And today JWs still teach this as a fundamental truth.
Thirdson