I think it is possible to do what you need by editing/copying the Performance Manager you've just created.
Please check the BMC Performance Manager Portal Monitoring and Management guide http://documents.bmc.com/supportu/documents/01/54/440154/440154.pdf chapter 5 Performance Managers and Application Classes, specifically page 156 and following "Editing Performance Managers". When editing a performance manager you should also be able to modify how parameters are displayed and add your own calculations. I remember doing this year ago, but I haven't played with the Portal in a while. Please give it a try,