Tracking spent time on related WI's

Hi,

I'm trying to track the time spent performing tasks in an hieratical WI schema like: WI-A > WI-B > WI-C

Example:

- WI-A -> contains WI-B -> contains WI-C
-> contains WI-C
-> contains WI-B -> contains WI-C

- WI-A -> contains WI-B -> contains WI-C
-> contains WI-C

What i need is (without using a plan) that when i check the state of a WI-A (Top level WI) know the % completed of it. But this completation % has to be calculated by:

(adding the time spent in the lower level WI's)/ (adding time estimation of lower level WI's.)

Any idea if this is possible? And where to find info about it? I've tried many OOTB relations between WI's but none has accomplished the behaviour I wanted.

Thanks!