For precise listening level you need to know your speaker/headphone's FR curve, combine that with your DSP FR response, and add A/C weighting filter, and do finally do RMS computation. This is just too much work and not possible on RME.
If you just don't care about precision, why not use headphone power calculator so you can quickly estimate upper limit of your volume position. Never turn it above that limit.
or, if you want something fancy, why not installing a calibrated mic (or just use an iPhone App) to measure it realtime when you are listening, and using software to automatically attenuate/stop music when it reaches certain level?
Last but not least.... Life is short, just enjoy your music. All great musicians suffer from hearing loss. No big deal:-)