How do you report hours that vary week to week?

I work some days through a staffing agency while still certifying. It has been weighing on me all week. 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.

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.