Here's a simple but useful expression when you want to link a Source Text to a Slider Control with two decimals and a percent sign after the main numbers. This is very useful when you, for example, want to create the counting numbers of a progress bar. Please note that you'll need to replace the slider reference-part (X) with your own slider reference. You can also change the number (amount) of decimals by changing the Y value.
Here's a quickly-created ugly example:
X = thisComp.layer("Controls").effect("Slider Control")("Slider");
Y = 100;
Math.round(X*Y)/Y + "%"