Sorry - bit of a yes/no question: Is it possible to make the slider from a manipulation jump in irregular steps? Or would the best way to overcome this be to just create a series of buttons that jump the manipulation to the desired position?
Just writing down what I and Rojo talked about in the comments, we get:
where the slider jumps from 1 to 5 to 20. The gap between one and five is four, the gap between five and twenty is fifteen.