Weight the can. Use what you have to to get the job done and collect the dust. Add the powder back in to the can and mix. Filler is just epoxy and talcum powder for the most part so it will still "work".
Claim the portion of the can that you used by weight.
As I said, I think you blew a creative rules interpretation opportunity by putting it on the table so that it could be litigated. It might have been banned next year, but I bet it would have been allowed this one time.
