Custom Wordpress Plugin

Custom Wordpress recipe and ingredient calculation plugin required.

The plugin is for a food website that will feature a number of recipes aimed at chefs/caterers. The purpose of the plugin is to allow users to enter the number of people they are cooking for and the quantity of each ingredient required will then be updated accordingly.
As an example please take a look at http://tastykitchen.com/recipes/main-courses/red-curry-in-a-hurry/
This is very close to what we are looking for, although we would not need the option of U.S. values, just metric measurements. It seems to be based around this plugin http://wordpress.org/extend/plugins/wp-recipes/ with modifications.