There's a difference between relative value (in the poll) and absolute value (to be fed into program)
Here is how it's calculated for OpenGL:
amb_user = (float)((Cmdline_ambient_factor * 2) - 255) / 255.0f;
// set the ambient light
GL_light_ambient[0] += amb_user;
GL_light_ambient[1] += amb_user;
GL_light_ambient[2] += amb_user;
The default GL_light_ambient[] values being 0.8, 0.8, 0.8, unless it's set to something else by the mission.
So by that, an ambient factor setting of 127.5 will do nothing (ie, 100%). A setting of anything below 127.5 will scale the ambient light level down. A setting above 127.5 will scale the ambient light level up.
I think the entire question is silly and ambient light should be settable in FRED.
Having it as a CLI destroys the opportunity for varied lighting in missions.
Ambient light IS settable in FRED. The cmdline option just sets the user perference as to the scale of that light. And that should NOT be a FRED option, under any circumstances.
The user specified ambient factor is required to deal more appropriately with video card differences, monitor/environment settings, other cmdline options, models/maps in use, and general user preference. You can't FRED around that stuff and it's just silly to think otherwise.