Well, decibels is a relative scale. In digital audio, we usually use 0db to represent the maximum amplitude (loudest, full-scale) sound possible a digital system. Then we count down in negative numbers from there. So -6dB is half of the maximum, -12dB is half of that (1/4), -18db is half again (1/16), etc. Total silence is -infinity.
The decibel scale is also used in the real world to describe the absolute amplitude of real-life sounds in the air, as they reach your ears. Is this is what you are looking for? When the scale is used this way, 1dB represents about the quietest sound your ears can detect, and we count up using positive numbers from there. Very loud sounds approach 100dB. 120dB becomes painful.
Really the only thing these two uses have in common is that every 6dB increase represents a doubling of amplitude, and every 6dB decrease represents a halving of amplitude.
In DP, when you see a meter showing -12db, that means that the sound on that channel is at 1/4 of the maximum amplitude for the system. (actually DP uses floating-point math internally, which can handle sounds over 0dB, at least until the final audio output.)
What this doesn't tell you is how loud the sound was in real life before it hit your mic. It might have been 40dB, or 80dB, we don't know. It all depends on how sensitive your mic is, how high your mic preamp was turned up, etc. All we know is that after your mic, and preamp adjustments, and A/D conversion, the resulting signal was about 1/4 of full-scale.
To determine the real-life sound level, you need some other point of reference. Most commonly this is a SPL (sound pressure level) meter, a tool used for measuring real-life sound levels. These are factory calibrated to give you an absolute reading.
If you have a SPL meter, you can use it to adjust ("calibrate") your preamp. Measure a long, constant volume sound with your meter. Let's say it reads 60dB. Now adjust your preamp so that the level meter in DP reads -20dB. Now you've established a reference. Whenever your DP meter shows -20dB, you know the real-life level at the mic was 60dB. If the DP meter shows -30dB, you know the real-life level at the mic was 50dB, etc.
Another way to do this would be to get the specs on your mic, find out how many volts it puts out at a given SPL, and measure the output with a voltmeter. But you need to know how to hook it up correctly, and also measure the ac audio waveform correctly, plus do some math using logarithms. That's why they make SPL meters.
BTW, there is no "inaccuracy" involved in using a mic preamp. You simply adjust the preamp so the recorded levels you get are in a reasonable and desired range. You never want the incoming signal to get up to 0dB, and you probably want it to be at least -40dB on the quiet stuff, as a very rough suggestion.
Now, if you're recording multiple tracks through different preamps, you might care if the preamp levels are the same. You can do this by playing a constant tone simultaneously into both preamps, and adjusting both preamps until the output levels match.
