You need to modify the nvram file.
11b power is set in qdbm units by the maxp2ga0 line. eg: maxp2ga0=68 sets +17dBm 11b output power. Whether this matches what you see externally will vary (especially radiated), but you can adjust it relative, eg if you want 1dB lower use (68-4)=64.
The 11g powers are set relative to this, as 0.5dB backoffs per modulation, one nibble per modulation.
means all OFDM modulations are backed off 2dB (MSB = fastest, ie 54Mbps, LSB = slowest, ie 6Mbps)
The 11n powers are set in two 16-bit values, but the concept is the same.
means 65Mbps is backed off 3dB, the others 2dB.
So to decrease 11b by 1dB but not alter the 11g/11n powers you would use (in this example):
(ie, overall power down 1dB, all backoffs reduced by 1dB)
does this answer your question? Let us know if it works.
Thank you very much for your response. Our problem seems to be solved by this new nvram file.