Sum estimates from sub-tasks in user stories

As a Greenhopper Scrum product owner, I want story estimates to automatically total to the sum of the estimates for all sub-tasks IF and ONLY IF subtasks are entered (else I want story estimates to be manual entry) so that I can view on the scrum board the original estimate against the remaining time.

Definition of Done:
1) When no sub-tasks exist for a story, the estimate field for the story allows entry of a value manually
2) When one or more sub-tasks exist for a story, the estimate field for the story is calculated as the sum of the estimates for all sub-tasks for that story

Context:

I'm finding a feature in Greenhopper to NOT operate as I would have expected. The feature is in estimating where the estimation on the scrum board is configured for original time estimates. What I would like is:
1) when no sub-tasks are created for a story, the estimation and remaining fields are editable within the story
2) when sub-tasks have been created, the story estimate and remaining fields are calculated as the sum of the estimate and remaining fields for the sub-tasks

What's currently happening is that the story estimate field is not calculated from the sum of the sub-tasks but the story remaining field is. Worse, if you enter a value in the story estimate field when sub-tasks have been added and estimated, the value entered into the story estimate field is added to the story remaining field, which already has the sum of the sub-tasks. Thus, if, as a work around, you try to manually enter a story estimate that equals the sum of the sub-task estimates, the story remaining field value doubles. There's no work around for this. This feature seems poorly defined. I understand what the product team was trying to accomplish... the ability to estimate and view remaining values for stories that have no sub-tasks or prior to sub-tasks being added. What they missed is that the field could serve both functions - manual entry when no sub-tasks exist and calculated value when sub-tasks do exist. I strongly urge the product team to consider this feature at their earliest opportunity as the inability to see from the scrum board the original estimate against the remaining value in plan and work mode is a problem.

Steps to reproduce

1. Configure Scrum board to use "Original Time Estimate" for Estimation Statistic
2. Configure Scrum board to use "Remaining Estimate and Time Spent" for Time Tracking
3. Create an issue with issue type Story, without specifying any Original Estimate and Remaining Estimate
4. Create 2 sub-tasks for this issue.
   • Sub-task 1 has Remaining Estimate of 4 hours
   • Sub-task 2 has Remaining Estimate of 2 hours
5. Checking parent of sub-tasks in Issue View, Original Estimate and Remaining Estimate is now shown as 6 hours
6. Navigate back to Scrum board, open Issue Detail View, and Original Estimate for parent issue is Unestimated