Suggestions for features OR Help?

Hello All,

I'm fairly new to Org-Mode, but common to Emacs.  I've have a need to
create a task which is made up of sub-tasks.  Each sub-task in turn has
actions which comprise it.

Each action, I'd like it to have a checkbox so that the sub-task can
track percent complete.  Like so:

  * Sub-Task [66%]
    - [X] Action 1
    - [ ] Action 2
    - [X] Action 3

Once the sub-task is complete, I can set it's TODO label:

  * COMPLETED Sub-Task [100%] - 2008/08/08 17:33:00

This is already how Org-mode works obviously.  My question/request is
this: is there a way to also track completeness via percentages for the
sub-tasks (headings)?  So I would have/see the following:

 * Primary Task [65%]
  * COMPLETED Sub-Task 1 [100%] - 2008/08/08 17:33:00...
  * Sub-Task 2 [24%]...
  * Sub-Task 3 [72%]...

The primary task percent would be based on 3 sub-tasks of equal weight, so:
  (1*.33) + (.24*.33) + (.72*.33) = 65%