Yup, the sliders get very sensitive when dealing with proportions, right down to the hundreths of a percent. So here's a tip that may come in handy - you can use the slider until you've "narrowed down" the target area you want, then type numbers to change it more subtley in small increments. Like moving from 30.03 to 30.07, for example. And as Mike said, the settings will vary with the specific terrain. So if you save the material and then apply it to a new terrain, it'll probably need some adjustment to suit it well. :)