Hi Jim
Having set me a challenge last night, I just had to try to crack it, and crack it I have

Attached is a simple model that has your "Tickover knob" This works up to 50% throttle.
When a tickover is set, the whole throttle stick range now fits into the remaining movement between tickover and full.
I've used Ch10 as a place to set the range and offset of the tickover knob. (This is then used in two places)
Ch11 is the inverse of Ch10 and the offset means it starts up at +100 when Ch10 is zero.
Global Variable 1 follows Ch11 and sets the weight of the throttle stick.
So, the throttle channel works off the throttle stick over a range of GV1
Added to this is the Knob S1 (range adjusted from 0 to 50%)
GV1=100, minus (Knob S1 (range adjusted from 0 to 50%))
Cheers
Chris