How do you report hours that vary week to week? - 5 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. Pay posts on a different day from when I work. Did anyone get an overpayment notice from a mistake?

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.