If I understand correctly, the problem you're describing is that you cannot actually rotate the model by 90 degrees by only controlling the Z axis, and the solution is to have the slider control the Y axis (or is it X?) as well to make sure that the camera goes in a circle around the model, with sin(Y) = sqrt (1-cos(Z)*cos(Z)) or some such equation.
I hope that that's indeed the problem you meant
