My grandma did the same thing with a regular soup spoon from her old silverware set, and she always said the scoop mattered more than the spoon itself. She'd fill it heaping and level it off with her finger, and her coffee came out the same every single morning for 30 plus years. The thing that gets me is she never once used a measuring cup, just that one spoon and muscle memory. I bet your mother in law could pour a perfect pot with her eyes closed by now. Did she ever try to teach you her exact count, or did you just watch and pick it up on your own?