By creating a calculated field similar to the following should get you the desired outcome:
Then, depending how the view is set up, it may be necessary to change how the calculation is being computed (right click on it and select Compute Using...)
Hope this helps!
Tracy, Thanks for your response but I don't think that's what I'm looking for. Here's an example of my scenario.
I have a total of 4000 orders; 218 in January 2012, 540 in February 2012, 459 in March 2012, and so on. How do I set the formula up so I can calculate the percentage in a month of the total orders? I want to chart the percentage of totals orders.