Where a state permits it, the split has to be supported by actual records — time cards, job costing, or a daily log showing how hours broke down. An estimate or a verbal description does not qualify.
Without supporting records, most states require the entire payroll to be assigned to the highest-rated classification the employee worked in.
For businesses whose staff move between trades, this is usually the largest single swing available at audit, and it depends entirely on records kept during the year rather than reconstructed afterward.