Need help with utilization report showing a column for billable time

New to Netsuite, and in process of learning while doing.  I created a utilization report for employee time,  this shows entered time, exempt time, and provides percentage calculations for each employee.  I would like to add another column that shows a percentage of time that was entered which is marked as billable.  Example, John Smith,  Productive 88%, Billable 35%.  I believe I need to create a field that will sum the total billable time and then have that show up in my available item list in the report area.  Any pointers would be appreciated.