If you change Duration for a task that is partially complete the Effort Remaining also updates, but not vice versa. Changing the Duration should not update Effort and changing Effort should not update Duration, the two should be independent. So the current behavior is inconsistent and just half right.
There are also weird automatic updates, for example:
- Create a task with Duration 1 week and Effort Remaining 10 hours
- Change %Complete to 50% -> Effort Completed & Remaining update logically to 5 hours each
- Now change Duration to 2 weeks -> %Complete changes illogically to 25%
- Now change %Complete back to 50% -> Effort Completed bizarrely updates to 6.67 hours and Remaining to 3.33 hours.
Furthermore, the default effort is also calculated for a task based on a fixed 8 hours/day which is almost universally unrealistic. You should either be able to set the benchmark hours/day based on assignee or project, or just avoid calculating a spurious value and leave the effort to be set manually.
Right now our organisation has given up entirely trying to use the Effort fields, since the behaviour is so unpredictable and unrealistic.