In theory, managed descent takes into account all the known variables and constraints and works everything out to get you to the initial approach fix at the appropriate speed and using the minimum fuel (according to your cost index) - but of course you have to have programmed in all the variables first.
As long as you have put in all the descent winds, and realistic speed and height constraints, it should get it right. The managed descent speed is variable between the 'hockey sticks' to allow for differences, such as stronger/weaker headwinds etc. It obviously cannot predict ATC requirements such as headings or speeds. From memory, our A320's usually idle at around 35-37% N1 in descent. Selecting engine anti ice on should increase idle slightly.
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Last edited by Uplinker; 17th May 2013 at 11:04.