Adobe Premiere - internal 5.1 to stereo downmix

Hello.

I would like to know what is the exact process / formula behind internal downmixing of channels, from 5.1 to stereo.

When exporting a clip with 5.1 audio to stereo file, using a ITU standard channel levels set manually in the mixer, the resulting peak level of a stereo file is much higher then the one where the mixdown levels are handled internally by Premiere.

Does somebody know what is the internal mixdown formula? Maybe somebody from Adobe? 

Thank you,

BR,

Simon