Return loss is the relative power of the reflected part of the signal, and the remaining power gets through to the antenna. For your case, the reflected signal is 30 dBm - 12 dB = 18 dBm. To get the final transmitted power number, you need to convert those powers to watts, subtract, then convert back to a dBm value.
30 dBm = 1000 mW (incident power)
18 dBm = 63.1 mW (reflected power)
(1000 - 63.1) mW = 936.9 mW (transmitted power)
936.9 mW = 29.7 dBm
For dBm <--> mW conversion see here:http://www.rfcafe.com/references/electrical/mw2dbm.htm
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster