shipping based on weight

Hello,

I need to pay 20% of the total cost of the shipment date from the module that I vied shipping based on weight, then I duplicate flat form for shipping and I'm changing.
Now I have a problem I can not figure out how to fetch the result of the expenditure from the cart I opened the source weight.php and I found this string $test = $ this-> cart-> getWeight (); Print me in my form twofold the weight of the neck, but I can not understand where is the result of the cost that he Print I then screen the cline to be able to run the 20% discount

Can you help me?

Thank you.

as it is called the string that shows me the amount of shipping based on weight in the checkout?

thanks