Paolo, I've been measuring what Synchron Player's Main Volume and Expression faders do, but haven't finished that yet - in every measurement it can take ten minutes or more until the meter readout is settled to 2 decimal places. (I'm using BlueCat's DP Pro Meter plugin, which does lots of very useful stuff, including RMS level averaging in real time over however long I want.)
A few highlights of my findings so far:-
Both faders have the same logarithmic transfer function, as one would expect.
-1.0 dB is at MIDI value 120; (all dB values here are relative to 0 dB attenuation at MIDI val 127.
-2.5 dB at 110;
-4.1 dB at 100;
-6 dB at 90
-8 dB at 80;
-10.3 dB at 70
Going all the way down to -83 dB at 1, then complete cutoff at 0.
If both faders are set to 90, the combined result is of course -12 dB. All combinations (excluding zeros) are simply the sum of the two dB attenuation values as yielded at the two MIDI values. There is no easy way of avoiding having to know what dB attenuation is yielded at what MIDI values on these two faders.
Sorry to say your heroically bold percentage idea can't be applied to any other combination except the one you've derived by listening or by meter readings. And as you mentioned, the simple but somewhat fussy mathematical expressions that can derive MIDI value from required attenuation dB, and attenutation dB from MIDI value, just aren't the sort of thing that mix engineers generally would care to memorise or even look up, let alone use in the heat of the moment while mixing.
What I want at least for myself is a complete and accurate chart of attenuation dB versus MIDI values - and I hope it may also be useful for you and many others. That's going to take a while because doing these measurements takes forever and certainly ain't fun. And I must say I'm a wee bit miffed that VSL haven't seen fit to provide any such chart, table, map or graph in the Synchron documentation.
Also I want to produce a simple drawing that shows several key dB values on one side of a fader and MIDI values on the other, linked by horizontal lines - as a sort of graphical template I'd like to see (or at least imagine) overlaid on these two faders in the Synchron Player.
The particular use case you outlined at the start of this thread is a critical one; it highlights a problem we all have to face if, like you, we want some default headroom on the Expression fader and yet also want to preserve the Synchron Player's preset overall output level. As I've quoted above, 6 dB down is at MIDI val 90. I'm worried that in some presets it might be impossible to raise the CC7 fader by 6 dB to compensate for this (not unusual) 6 dB of default headroom on the Expression fader.
One workaround might be to set the required Expression headroom of 6dB in all Synchron Players in use, but leaving the Volume faders untouched - then simply make up for this 6 dB loss on all the Synchron Players, at some common stage downstream.
I'll post the full results of my measurements probably in a few days.