Each persons washer and water is different so the amount of detergent you need to use is different. You also need to consider how dirty your laundry actually is and what the fabrics are. Following the manufacturers fill line on the cap or scoop is generally wasteful overkill. Do some experimenting of your own to see how much you really need to get a load clean.
I have found that most of my loads get just as clean using a table spoon or two of liquid detergent as they did when I filled it to the line. I often found that using the recommended amount of detergent made my washer outlet fill with bubbles. What a waste. Also remember that if you use a stain treatment like Shout or Zout they are basically concentrated detergent so a load that has stain treated items will generally not need as much detergent. I have also been known to coat a big stain with scrubbed in detergent to get a stain out if I need the item to get out of the wash quickly and it is not to tough of a stain. Stay tuned for recipe for a great stain treatment that gets it all out and inexpensivly.