10.6 is not good.
VNAV used to calculate idle descents but with 10.6 it only does this until the first altitude constraint. After the first altitude constraint it will calculate a descent with a constant angle (a non idle descent) instead of programming a level segment and then descending in idle.
If you fly level this will look to you as being high on the profile, while you know you are not.
You can work around it by programming a new cruise level in the crz page, dont forget to change the cruise speed and descent speed again, the fms will then recalculate a new idle TOD. And you are no longer to "high" on the profile.
Or just ignore it.
Tell your students to turn the meters indication (altitude) on. This will divide the altitude approx by 3. (only on the NG)
Since the height/3 + 10 is the distance in miles you will need for descent.
They can instantly compare the height to the trackmiles. (prog page)