connecting two entities using ribbon workbench button

Hi,

I have placed a "button" in ACCOUNT entity using ribbon workbench.

In ACCOUNT entity - create field - "Total Tax"

In CONTACT entity - create field - "Tax"

Requirement -

I have to write a javascript code to achieve the following..

In contact entity if i create 4 records with some values in custom field "Tax", then the total of all the "Tax" fields in all the 5 records should get summed up and  updated in "Total Tax" in Account entity...