Dialog Normalization Question

Noob question for sure.

Exporting my first 5.1 mix in STP and I'm confused by the Dialog Normalization field setting. STP manual states "For example, if the dialogue in your project averages around -12 dB on the level meter, enter -12 in the Dialog Normalization field."

Then it says "If you enter -31 dBFS, the attenuation is 0 dB (31 dBFS–31 dBFS), and the source audio levels are not affected at all."

How can both be right? If my dialog (Center Channel) averages around -12, do I enter -12 or -31 into the Dialog Normalization field?

One other note that is potentially the reason for my dilemma. This show will be screened from blu ray or DVD in a theater. I sent out test discs last week and the projectionist said his board was reading -39. Is this because Normalization was set to -31 for that mix? Did I just answer my own question?