How do you report hours that vary week to week? after 3 years as a bank teller

I work some days through a staffing agency while still certifying. 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?

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