How do you report hours that vary week to week? - 11 months and counting

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I have been trying to stay organised about it. The form asks for gross earnings, which I do not see until later. Did anyone get an overpayment notice from a mistake?

The claimant handbook has worked examples for part-time pay. Mine was clearer than the phone line.