I recently put together a simple web-based volume output calculator based on this forum post by tinyman392, and though I'd share it here. You input your headphone impedance (ohms), headphone sensitivity, the max voltage of your audio source (volts), and the volume as a decimal (e.g. 50% volume = 0.5). Please note that this is only designed to give you a rough estimate, and It assumes that your volume increases at a parabolic rate (which it should). Enjoy!