How do you report hours that vary week to week? (update)

I work some days through a staffing agency while still certifying. I worked as a copywriter before this. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 7 months ago, and it settled down once I kept a simple log of every call and form.

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