Mapping MIDI Data to Z3TA

I usually turn to MIDI Learn over z3ta+2 Main Volume, as there's more control over the entire range: -inf to 0 dB.  The Modulation Matrix is additive/subtractive over the targeted parameter.  In other words, you'll be adding or subtracting here to wherever your Main Volume is currently set.
 
Set your Main Volume to around - 20 dB.  Some signal will leak through with a CC #7 value of 0.  You're adding to - 20 dB.  Then put in a Modulation Matrix slot such as:
 
Source: ON
Range: (fully right)
Curve:  PITCH 4 OCT
Control:  CC7 (VOL)
Destination:  MAIN VOLUME
 
The key here is the Curve entry.  It's an old z3ta+ trick to 'amplify' the input and map it to a larger range of the destination.  If you need more range of control, duplicate the Matrix slot above in a 2nd slot.  You can enter a 3rd or 4th slot, if required.  They all add together.
 
Just remember to lower the Main Volume control to -35dB or -40 dB before a 2nd slot.  Adjust the target control (Main Volume slider) to a lower 'base' setting every time you're going to add to it with another Modulation Matrix entry.  Just don't bring it down to the area near -inf.  You're dealing with a logarithmic scale and decibel ratios.