How do you report hours that vary week to week?, procurement assistant here

I picked up part-time shifts as a procurement assistant and I am confused about how to report them. I would rather hear the honest version than the polite one. 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.