Kari, I don't think anyone is suggesting getting a calculator out and working out the engineering calculations relating to photography, since the 1/FL or 1/(2fl) is more of a mnemonic, to help visualize a simple relationship. It is not meant as a rule but a reasonable starting point from which one can deviate as the conditions allow.
If someone DID want to calculate everything that the camera considers, it would be very much akin to rocket science. The more you dig into the complexity of the systems involved, such as AF systems, the more in awe one would become of the engineers and programmers at Nikon. There is more processing power and precision in a D800 than in the Apollo moon shots with very complex algorithms being employed in a couple milliseconds. It is moving vast sums of data, acting on the data and storing the data at levels only dreams of by rocket scientists of the era. The calculations of the sense transducer output before the shot is even more complex. The color temperature of light for example changes all the calculations to compensate for the difference in refraction index by color, so focus needs to be biased. There are hundreds of similar processes going on in that 50ms between pressing the shutter and the capture taking place. So, given that background, having a couple easy to remember overly simplified memory aids for mere mortal photographers like us is welcome. Stan St Petersburg Russia