Estimates in parent tasks and subtasks

At the moment the estimated time from the subtask is always added to the parent. But this is confusing and misleading. There is a "hacky" solution (https://www.easyredmine.com/component/easyredminehelper/?view=knowledge_detail&id=193&category_id=9) to get a better insight, but this is only for each task and causes problems if a task doesn't have subtasks.

There should be a feature & setting for each project, if the estimates in subtasks should be added or calculated against the estimates in the parent tasks. And only, if there are Subtasks. 

Usecase:
In the parent task are normally the whole estimates, done e.g. in the sprint planning. Team members are now splitting the tasks in subtasks and add the estimates for the subtask. The estimates of the subtasks are some percent from the estimates of the parent task.

Current Status:
The estimate will be added to the estimate in the parent task. In the end we have the following constellation:
parent task: 140 %
subtask: 40%

How it should work:
The estimate should not be added. There should be also some field where you can see the difference.
parent task: 100%
subtask: 40%
open: 60%