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

I work some days through a staffing agency while still certifying. There are 15 people counting on me, so I am trying to be careful. Pay posts on a different day from when I work.

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

Keep everything in writing and save the confirmation numbers. It saved me later.