Suggestion for the timing slider

When adjusting the timing (advance / retart) the default is '50' right in the middle of the slider. I have been assuming this to be 0* TDC.
Please correct me if I am wrong.
If I am correct, I would presume that '100' correlates to a certain degree after TDC, and '0' being a certain degree before TDC. (it could be 50* in either direction but I do not know how this was programmed / calculated). I think that having the slider at default, in the center, being '0', full advance being +50 and full retart being -50 would be nice. I know its semantics, but just a thought.
Please correct me if I am wrong.
If I am correct, I would presume that '100' correlates to a certain degree after TDC, and '0' being a certain degree before TDC. (it could be 50* in either direction but I do not know how this was programmed / calculated). I think that having the slider at default, in the center, being '0', full advance being +50 and full retart being -50 would be nice. I know its semantics, but just a thought.