shou should read more careful the configuration - that setting is used for the prism cards and not for the atheros card.
To determin what output power you have, you can use some power meter - lock to the lowest data rate and transmit with that card. Another option you could monitor the wireless interface when it is running from the console and you will see te current-tx-power. If that card isn’t using some offset then probably that values are correct tx-power.