I honestly don’t know what the default value is. I think it used to be 100 back in the Atari days…and that was determined by the fader’s default position in the GUI. The thing is, if you are setting the volume of an external piece of MIDI gear, that setting stays in place once you set the Cubase fader to “Off”. The external unit’s volume control now becomes independent from Cubase until you set to anything other than Off.
I guess what you can do is set the fader to some arbitrary position along the track that permits some headroom above it that sounds ok, then look at the external unit and see what it’s showing as it’s volume level. Whatever that number is, is your personal “default” level.
I have a ton of external synths and haven’t used Cubase to control their volumes via MIDI in years. I keep Cubase MIDI faders set to “Off”, set a max volume on the synth and do volume control with Cubase’s audio channels fed from the synth’s audio outputs. Much better resolution and control.
But that’s me. If you prefer to control volume strictly MIDI, try what I suggested in the 2nd paragraph. Start with 100 and see if that gives you enough volume and headroom. Pick your own “default”.