Custom Product Field - Needs Extra Functionality

[natalie123123]

Hi, I have successfully created a custom product field as per the tutorial here: http://nemops.com/extending-prestashop-objects/#.VrsHnNCX-SM

 

It is for the product 'volume'. What I'd like the system to be able to do is calculate the 'total volume' of the cart on the front end, dynamically (exactly as the core system does for 'total weight')

 

I've already customised Prestashop to be able to display the cart 'total weight' on the front end, so I know how to achieve this part.

 

Essentially I want to duplicate this functionaliy, and call it 'total volume'.

 

Any assistance would be much appreciated.