This machine is designed to deliver the correct brewing pressure based on the chosen coffee bean type and the fineness of the grind. When brewing, the indicator level should always be within the optimal zone.
If the indicator level falls below the optimal zone, this indicates an under extracted espresso and requires a finer grind to be used.
If the indicator level moves past the optimal zone, this indicates an over extracted espresso and  requires a more coarse grind to be used.

Please note: Different coffee bean types may result in a different optimal zone reading. Remember, as long as the indicator level resides in that zone you are getting the perfect extraction.