Try this:
Every time you empty your cup or change tampons/pads, make note of how full it appears and estimate mL lost.
Put that number in your period tracking app/chart or just on an old-school sticky note. Do this every time you change throughout that period!
You probably have a heavier than normal flow if you...
changed a half-full cup over 6 times
fully soaked 16+ regular tampons
fully soaked period underwear 4+ times a day for several days.
Menorrhagia is an excessively heavy flow. A few signs of menorrhagia are:
Blood loss of 80 mL or more during a cycle
Periods lasting 8 or more days
Clots bigger than a quarter
Bleeding full through pads or tampons within an hour