The basic rule about mixing with headphones is DON'T. There's an old saying the "Headphones will break your heart". There can be issues with levels, reverb and panning when mixing on headphones and then listening back on speakers. However if that's all you have, that's all you have. There's no way to calibrate phones easily, unlike speakers. But, to start, get this: http://www.atsc.org/refs/a85/Speech_sample.wav
That is a sample of speech that has been carefully chosen and measured to read -24LFKS. Adjust your headphone volume until the Dialog feels "comfortable" or "right". This is then your "reference" level. Not perfect but will get you in the ballpark. Be sure before you actually deliver you do take the mix SOMEWHERE to listen to it on speakers. And be prepared to make some changes...
I'll also point you here : http://www.soundonsound.com/sos/jan07/articles/mixingheadphones.htm#7
A good article on various plug-ins that help alleviate some of the issues when mixing on phones. Again not perfect, but will help.