Is it safe to listen to Apple earpods at 75% volume?
While this is a little bit off topic, I'll provide some thoughts here.


you can't expect the percentage scaling directly with dB SPL. While both are surely nonlinear inside the affected electronics, the nonlinearity of Apple's percentage may not be logarithmic but dB SPL are. 
sensitivity is dB SPL at 1 V. This usually has not much to do with the sound pressure level, your headset produces at 100%. 
audio files and sounds from apps and programs may be scaled differently. Computer equipment and phones usually do not provide a compressor algorithm, so a setting of 75% yields totally different sound pressure levels for different sound sources. 


Conclusion: No, it is not necessary safe to listen at 75%. It may or may be not healthy.