Without a scale one can't definitively weigh the black sands, particularly the stuff that was stockpiled weeks or months earlier.
One could perhaps get a rough estimate of a days work based upon an hourly through put, but that doesn't do much good when estimating months of accumulation.
I don't see how that was such a bad answer, although it was good of you to "cut him some slack" over it if you think there was a better response possible.
Without a scale, how would you determine how much was there?