What functions or calculation can I use to separate a Total Duration into Straight Time hrs and Overtime hrs?
Assuming Duration is a time field, and sTotalDuration is a summary field = Total of Duration, try :
Straight Time =
Min ( GetSummary ( sTotalDuration ; Date ) ; 28800 )
Max ( GetSummary ( sTotalDuration ; Date ) - 28800 ; 0 )
Result is Time for both.