Whatever the level you have the meter set at, "0" is always going to represent that level.
But on mine, 0 and the 12 o'clock position are one and the same......unless you're talking about the digital model.
If you have it set to 70dB then if you have a 70dB tone I believe the meter should be in the 12O'clock position. A swing to the left indicates a drop below 70 and a swing to the right indicates a rise above 70.
Of course, I don't have mine right in front of me so I could be mistaken.