Antenna gain is depended on what kind of antenna AND more important cable losses.
You should always leave at 0 db from the antenna itself.
So if an antenna has a gain of lets say + 12 dB, and u use a cable which has a total loss of -15 dB, then your gain at the AP should be +3 dB to compensate gain and losses of the total setup.
Be carefull with gain settings, and calculate before making adjustments. When your gain at the AP is too high, then radio's using the AP which are nearby make the incoming signal too high, what then leads to packet losses of corruption.
Simply stated: Less is more
