Chris,
You have to set both percentages to 100% (or -100%) to get the servo to operate in both directions. Move the elevator stick up and down to switch between the 2 settings.
As said above, this is NOT an ideal combination for your model. I would really recommend a servo reversing yharness or a servo synchronizer instead. Much safer way to go.
Sincerely,
AnnMarie Cross
President
Crosswinds Equine Rescue, Inc
http://www.crosswindseqresq.org/