Calculation Unit Percentage

I want to add an item to my rates that is a percentage of the cost. For example, lets say I am charging $0.50 per word for Spanish translations and for proofreading, I only want to charge 50% of the cost per word ($0.25). Is there way to use the percentage calculation unit to do this? or what is the best way to do this? 

In the past, I have just added a proofreading charge for the languages needed and done the math myself and charged half the cost but this can be time consuming and I am looking for a more efficient method.