The correct setting in my case is 96% and not 100%.
# amixer -c 0 contents numid=1,iface=MIXER,name='PCM' ; type=INTEGER,access=rw---R--,values=2,min=0,max=215,step=0 : values=207,207 | dBscale-min=-103.50dB,step=0.50dB,mute=1
Note that at step 207 we are at 0dB (-103.50dB + 207 * 0.50dB = 0dB).
The playback volume should indeed be 100%, which corresponds to 0dB, or 2VRMS (http://www.ti.com/lit/ds/symlink/pcm5122.pdf, page 6). I’m not sure what the playback gain actually does, except for providing an additional 0.80dB analog boost (page 99 in the pdf). It might be at the expense of something else. The default setting is 0, so I leave this one off. It’s a very small difference anyway.
In my case 96%, 100%, 0% is indistinguishable from my other line output devices. PCM/digital at 100% is clipping the digital signal. But please check it for yourself with amixer -c 0 contents, because I was using another RPi distribution. The device number might be different too, so check that first.