Progress lines at summary task level

[MP]

Since duration for a summary task is calculated differently from duration for
a subtask, sometimes when I use progress lines, at summary task level
progress line can slip to the rigth, although ther isn't any subtask ahead on
shcedule.

I want project team to associate the rigth slipping of the progress line as
project being ahead of wath is was supposed, but this is not always the case
at summary tasks level.

Thanks in advance,
Marta