Can’t be easy with all OT gain stages!
GAIN 0 (Mixer)
VOL +63 (Thru only)
GAIN 0 (Pickup only)
VOL 0 (Amp)
FX1 (beware*)
FX2 (beware*)
LEVEL 127 (Default is 108, around -3db)
MAIN LEVEL 0 (impact on SRC3 MAIN recording)
*FX can increase levels drastically, especially the Compressor! 31 GAIN is around 24 db, I didn’t go further for test. It can be interesting for very low signals, as a guitar or dynamic mike straight in OT, with all other parameters maxed! 
The values are a reference for unity (what comes in = same ouput level). I liked @philtipping approach / diagram, ( Signal flow/gain staging diagram for Octatrack MKII).
I tried to make a simpler version (at least simpler for me), not sure it suits everyone!
Octatrack-MKII-Gain-Staging_will5.pdf (170.4 KB)