Set color stop position when using advanced gradient fill.

When using the "Advanced Gradient Fill" fill option, in keynote or numbers for example, is there a way to set the position or to "center" a color stop? You can add a color stop by clicking with the mouse anywhere along the gradient but this isn't very exact as it just puts it at the mouse position. I want to be able to place one in the middle, or a third along the way. Do I have to line my cursor up pixel perfectly to do this? Optimally there would be a way to set a percentage position or something like it but I haven't found any way to do this.