How do you report hours that vary week to week? - first time posting

I work some days through a staffing agency while still certifying. I do not usually post, so bear with me. 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.

The claimant handbook has worked examples for part-time pay. Mine was clearer than the phone line.