Here are some useful links.
http://www.sengpielaudio.com/dB-chart.htm
http://en.wikipedia.org/wiki/DBu#Voltage
But, on looking at it more closely, I have been guilty of a mistake myself, in that dBu and dBm are the same with a 600 ohm load impedance – I'd said that dBu and dBV were the same, they're not.
So, for your purposes, the 7600's meter readings are relative to +25dBu(m), e.g. -12dB translates to 25-12 = +13dBu.
Why do we do it this way ? My guess is that the amount of headroom is often more important than the absolute level – if you remove the – sign, the 7600 meters display the current headroom.
