Photograph the whole panel under the PDR light before you start. It records what was there, including old damage, so there is no argument at pick-up.
Mark dents with a pen before you work, and mark them off as you go. It stops you missing dents and gives you an accurate count to match the quote.
Keep the photos with the job.
Go deeper: this idea comes from the PDR eBook. See what is inside the eBook.
Prefer hands-on learning? See training dates. More PDR tips.