Yes, this works. The net effect is to shift *everything* one semi-tone. Ab = A, Bb = B, C = C#, Db = D, Eb = E, F = F#, G = G#. The salient point here is that all regular major/minor keys contain only 7 notes (the octave being a doubling of the root).

The rule might be slightly better quoted as "Sharps + Flats = 7" e.g. Ab (4 flats) = A (3 sharps); G (1 sharp) = Gb (6 flats), which is the same as saying F (1 flat) = F# (6 sharps). Only goes to show that some composers want shooting! :-/

;-)