Calculating a duathlon race pace

Try an on-line calculator.  Some will let you enter a time trial (i.e. 15 mile bike TT, or 5K run TT) and it will give you predicted times of other distances.  I just match the run and bike times to the entire time of the event (i.e. 4 hours).  I then use the pace from the entire time of the event to pace the run and bike legs.  It may get a 7:22 min/mi pace for a 4 hour run based on a 5K TT so that is the pace i shoot for on both run legs.  I may get a 19 MPH pace for a 4 hour bike ride based on a 15 mi TT (although you need to know what your HR, power, or effort is for that TT and convert it since time metric on the bike change with the course and wind conditions), so I shoor for a 19 MPH effort on the bike leg.