Redmine

Better to explain the situation with 1 example:

During project estimation, the project A is starting, we estimate to spend 100h in developing this group of features (es: Group API), 50h for that group of features (es: Group Server), 50h for a next group of features (es: Group DB).

Then we start to work.

Of course, strange bugs appears, maybe we can save 20h in the development of "Group API" but we spent 70h in the "Group Server", while we spent only 45h in the "Group DB" (so at the end we're happy, because in the overall project we saved 5h).

Question: how do I have to organize this?
Problem: a parent-task couldn't have his own "Estimated-time", as if I create a subtask the "Estimated-time" is zeroed...
Unclean workaround: At the moment I create another subtask called "bucket" in every parent-task (es: one in "Group API", one in "Group Server", one in "Group DB") with the estimated time for the group, then every time I add a real sub-task (for example, a bug is found), if I estimate it X hours, I should go and subtract X hours from the bucket.......... grrrrr!!!

Is there a cleaner way?
Or a hint on how I can make a simple patch... but which can be a correct approach to solve it cleanly?

thank you,
Filippo