How To Implement Custom Rollups for Dimensions

Hi,
I am interested in knowing how to implement a custom rollup depending on what dimensions are selected or set. When all dimensions are set to the lowest level, the values for the measure are displayed accordingly. However, if the dimension is set to all, the measure is aggregated.

I want to be able to see the min or max or avg when at a higher level in the dimension. For example, if I drill down in the country dimension to country group, I want to calculated the min commission rate for all customers for that country or determine what is the min commission rate for all customers or all country.

I have the following dimensions and measures...

DIMENSIONS
------------
OFFICE -> LDN, NY
PERIOD -> YEAR, QUARTER, MONTH
UOM -> BPS, CPS
COUNTRY -> COUNTRY GROUP, COUNTRY NAME
CUSTOMER -> CUSTOMER NAME

MEASURES
----------
TOT QUANTITY
TOT PRINCIPAL AMT
TOT COMMISSION AMT
MIN COMMISSION RATE
MAX COMMISSION RATE


CALCULATED MEMBER
---------------------
AVG COMMISSION RATE = (IIF([UOM].CurrentMember.Name = "BPS",
[Measures].[Total Commission Amt]/ [Measures].[Total Principal Amt],
IIF(UOM.CurrentMember.NAME = "CPS", [Measures].[Total Commission Amt] /
[Measures].[Total Quantity],NULL)))