How do you report hours that vary week to week? - any advice?

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I would rather hear the honest version than the polite one. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

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