It would be great to add the issue field advancement percentage for time tracking. This field could be updated in concert with the remaining estimate and time spent. For example, a task with 4 days of time spent and 6 days of remaining estimate may have an advancement percentage of 40%
(time spent / (time spent + remaining estimate)).
Also when logging work it may be used to adjust estimate. A developper could state that he's at 75% of advancement. For example,
Original estimate 4d
time spent: 2d (developper entry)
advancement %: 75% (developper entry)
With those entries, Jira could adjust the remaining time to 5.3 hours
(time spent / advancement percentage - time spent)

I propose that because our administrator like % and for them it's easier to see the overall advancement with % instead of time.