How Do I Know If I'm Over/under-filling My Machine?

What are the general rules when it comes to how many clothes you place in the machine for each wash?

I'm always worried that the machine isn't washing at maximum efficiency because I'm either over-filling the drum with clothes or perhaps not putting enough in. Generally speaking, I tend to loosely pile-in the clothes until they are a little under halfway below the centre point of the drum, thus leaving them plenty of room to circulate in the wash. I have seen other people almost fill the drum with clothes.