Change keyframes value using expression

Try this (I've taken one of Dan Ebberts' expressions and adapted it).   I also added in Layer Controls to make it easier for naming conventions and simpler to swap out layers.

var sliderValue = effect("Slider Control")("Slider");
key = sliderValue.nearestKey(time).index
if (time < sliderValue.key(key).time) {
	key--;
}

parseInt(effect("Layer"+key)("Layer").text.sourceText);