How do you report hours that vary week to week? - 2 weeks in

I picked up part-time shifts as a data-entry clerk and I am confused about how to report them. I have been trying to stay organised about it. 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.