Custom grouping of Pivot Table Data

[jldccc2007]

I have a pivot table showing groups of customer ID and dues against them as shown below,

First, the data has been sorted from highest dues to the lowest then grouped as first top 50 then next top 50 & so on, manually. I have used the pivot table because I wanted to get the due figures as a percentage of total dues for each group. Now I want to group them automatically following the above rule and want to know how could I do it. I couldn't find any solution for it. Any help regarding this would be a life saver.