Here's a Headphone Power Calculator spreadsheet:

You input your headphones' impedance and sensitivity (in either dB milliwatt or dB volt and it calculates the other sensitivity value). It also calculates the power and current at 1 volt rms. Input a target loudness and it'll give you the power, voltage and current required to get there using your headphones. You can also input an amplifier's Vrms rating and it'll give you the max headphone loudness and the power required to get there.
I hope you find this spreadsheet useful.
Rob Robinette
I just published a free headphone calculator for the Android. It has power calculator and resistor network calculator pages along with help pages. I'm still deep in the development phase so any suggestions for additions or corrections would be appreciated.
In the  Google Play Store search for 'headphone calculator' or Rob Robinette.