After selecting a milestone from the roadmap the information about expenditure of time is shown.
used time (if used by the project) is also shown from the time tracking information
IMHO a little controlling element could be added easily.
Redmine also knows how many ticket have been closed. It also knows how many tickets are not closed, yet.
The expenditure of time for closed and open tickets should be shown, too. having this information, a quick matching can be made, for example:
We know that 1 week is over now, 3 weeks are remaining for the milestone. 300 hours have been estimated once and tickets for 100 hours are closed already - so we know that we are in time.

You could display the information like this:

expenditure of time: 300.00 hours
closed tickets: 100.00 hours (33 %)
Remaining: 200.00 hours. (67 %)

The data should be available easily - maybe you also think positive about this idea?