Adding a NEW total in list page, as a combination of two totals

Hello from a very warm Italy.
With my PHPRunner I need to have in the 'list page' one total which is not the count/sum/average of the above records, but a division of two other totals next to it (I need it to calculate the weighted average).

I tried with PHP code snippet, and I also had a look at http://www.asprunner.com/forums/topic/19065-implementing-custom-count-function-on-the-list-page/ and http://www.asprunner.com/forums/topic/8847-operation-with-totals/ but I can't get a solution.
May someone help me?

Thank you so much
Matteo