The Cisco Aironet 350 cards are adjustable to 1, 5, 20, 30, 50, and 100
mW. I have heard that you have to do a reset between adjustments, however.
You can write a register on the baseband processor of the Prism 2.x
chipsets to adjust the power setting "goal." You read another register
to find out the actual level. I cannot find the units for the registers
in any document, however, the firmware may store some hints produced
when the manufacturer calibrated the radio. The firmware implements
an automatic TX level control, I think; you will have to disable that
before you make your own adjustments. I do not know the minimum TX level
for any Prism radio, but there are 100, 180, and 200mW Prism radios.