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

I work some days through a staffing agency while still certifying. I have read a lot of threads here and could not find my exact situation. Pay posts on a different day from when I work. Did anyone get an overpayment notice from a mistake?

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 made the follow-up calls so much easier.