Greetings matrixrf:
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:
https://www.rfcafe.com/references/electrical/mw2dbm.htm