Not quite, the fix is on the control input, so when you set a fixed value that applies to the input to any of the mixers that use that input. The mixers then determine how much of this output is sent to the control surfaces.
As for the mixer bars, yes probably in your case, have you used the offset feature to re-centre the flaps so you get more down movement for crow braking on the spoiler control?
Agree the manual could be more detailed, having come from a MPX 35 MHz Cockpit Sx and Royal Pro I understand the MPX philosophy, but if you are new to the system then the new manual doesn't explain this as well as the detailed manuals supplied with those sets.