Don't payattention to the value from datasheets. It is the maximum technical possible output power. The allowed RF Power (EIRP) depends on country channels and powerregulations. For example the max EIRP in Germany is 20dBm for 2,4GHz and 23dBmfor 5GHz.
At the beginningof configuration of AP you have to choose a country. After that the maximumallowed AP gain will be limited automatically. For R600 for 2,4GHz in Germanyit will be 17dBm due to the antenna gain is 3 dBi. (20-3=17). If you would liketo decrease the output power you have to configure an extra attenuation from 1to 10 dB. It means you can change the EIRP in my particular example from 10 to20 dBm for 2,4GHz.
The Ekahausite survey tool shows the EIRP also. And actually it is exactly what do youneed. Firstly increase the Tx Power until you reach the maximum allowed EIRP inyour country. And then decrease the Tx Power with a 1dB step to reach the poweryou need.
If you need an the half power out from AP, your EIRP should be 3dB less as max EIRP