Volts to decibels using microphone's transfer function
250mV/Pa is an RMS voltage of 250 mV for an RMS sound pressure of 1 pascal and this is usually specified at 1 kHz for microphones. 1 Pa is a sound pressure level of 94 dB. See this calculator.

So 250 mV RMS is what you get for an SPL of 94 dB. To convert 0.25 V to dBV you take the log then multiply by 20 to give you -12 dBV. If your pre-amp has 12 dB of gain then your microphone is producing -24 dBV for 1 Pa.

The noise level you specified (18 dB SPL) is 76 dB lower than 1 Pa but I'm unsure what relevance it has to your basic question.