Dynamic measures for drilldowns

All,

My hierarchy is as follows:

L1: main KPI

L2: sub KPIs

Example:

KPI: Quality

Sub KPI: vendor quality; material quality; customer quality

Approach I take now is as follows:

1) create separate measures for each KPI/sub-KPI level 

2) create 2 charts; 1 chart with KPI as a dimension in which i use yet another measure that based on KPI ID will calculate and display correct %; and 1 chart wiht sub-KPIs as a dimension in which i replicate the previous measure but at the sub-KPI level

3) selections in the KPI chart nicely displays related sub-KPIs

But, I would like to move towards the drill down function but my current measures are not displaying correct numbers anymore. KPI level gets calculated correctly while sub-KPIs get defaulted to the KPI numbers.

Any idea how I can solve this?

Thanks!