Request for simple Ofbiz calculation tutorial.

[Rasdi Ishak]

Hi. i'm new to Ofbiz . I have done the tutorial for beginners and try to build 
a plugin for sales commission and overriding. I have build the entity in derby 
and already successfully create and update a list. Now i am stuck at how to use 
java or groovy for calculating the sales commission, for example: commission = 
sales price * 85%. 
do anyone have a good example how this work? thanks for help.