I have an entirely different approach, and perhaps this is impossible if you want to keep the client on your books...but I would make the client CERTIFY on the census 1,000 hour, 500-1,000 hours, or less than 500 hours. Exact number not needed. Put the onus on them, where it belongs.
If a client is unwilling to make that decision and certify data, I'd politely show them the door. I certainly wouldn't make myself crazy trying to use an estimate that may or may not be accurate.
I caveat this by freely admitting that there may be other intricacies in your particular document/administration/situation that would make my approach impractical or impossible.