Well these are practical rules that are more or less used by people working with dBs. These people are usually don't think in term of bitfields - when you have

Well these are practical rules that are more or less used by people working with dBs. These people are usually don't think in term of bitfields - when you have a 43 dB gain antenna you don't care if it amplifies 20 000 times or 19 952 :) Also it's a nice way to show-off to people that do not know this rule! In any case, I never said that you get 100% accurate results; if you want accuracy then you should use your calculator (or your logarithmic ruler); but why use a calculator when you roughly want to understand how much a 15 dB gain would be? Finally, there's a nice way to find out how much 1 dB is with the mentioned rule: Notice that 1 = 10-3-3-3 thus it's 10/2/2/2 = 1.25 so 1 dB is approximately 1.25 gain as you said :)