Mute the notifications and set the desired ringtone volume. Simple and easy. I use it in work, I want to hear the ring calls but not the other notifications.
Since Android 4.0, Google linked by default the ringtone and notifications volume into the same bar. If you have a not default Android with 3rd party software on top (like HTC Sense, Samsung TouchWiz... etc) you are probably not having this problem, but if you have a "Pure Google" Android phone (like Nexus family), this app is for you.
This is an Open Source project. You can check out the source code at: https://github.com/jbilbo/SeparateRingtoneVolume