dbc to dbm
dBm and dBc are completely different so there is no 'conversion' between the two.
dBm - indicates the power measurement relative to 1 milliwatt. 0 dBm means no change from 1 mW, i.e. a power of exactly 1 mW. 3 dBm means 3 dB greater than 0 dBm. dBm is the relative difference to a fixed reference power of 1 mW.
dBc — indicates the power relative to the power of the main carrier frequency; typically used to describe spurs, noise, channel crosstalk, and intermodal signals which may interfere with the carrier. dBc is the relative difference to an arbitrary reference power level, the power level of the carrier frequency.