We chose to build the antenna for 2.442GHz, or channel 7, as this is close to the center of the 13 channels available to us (US designs use channel 6 (2.437Ghz), as they have 11 channels). I have included a key lengths calculator in the form below. Nb.Rectangular waveguides use a different formulae (see Rectangular Waveguide).
The final product only needs to be accurate to about 1mm as this will only shift the frequency within the range of the 802.11 spectrum. A smaller cavity length will shift the frequency response to that slot to a higher channel. A longer slot cavity length will shift it down.