Calculating average across multiple tables

Hi,

I am relatively new to WebI, so this question may be pretty basic. I tried some searching but could not find a good answer.

I defined four tables in a report, all are based on the same source query and have different filters

First table "Current" - Filter => clearing date is not null

Second table "Cross Table with groups" - Filter => clearing date is null

Third table "Past Due" - Filter => clearing date is null

Fourth table "Tota;" - No filters

I want to insert a percentage field as shown in the picture above that calculates the relative percentage of the value based on the total for January 2012.

How can I do this ?