Control Boss GT 1000 core output level with Launchpad Pro MK3

[ravmitterhoff]

I have my Launchpad Pro MK3 in midi custom mode set up to change patches on my Boss GT 1000 core and it works great but I cannot figure out how add a fader to control the output level (volume). The fader variables are only channel and CC#. How do I find the midi channel and CC# of the output level on the Boss GT 1000 core ?