Do You Know Your Flow??

So I'm glancing at the side of a box of Tampax Multipax tampons--don't ask--and what I read there left me a little perplexed. To protect themselves from being sued, there is a chart us ladies need to follow, apparently to keep ourselves from coming down with Toxic Shock. To reduce our risk, we need to make sure we are using the correct tampon that meets our absorbency needs. I always figured, hey--the first few days use the big boys, the smaller ones toward the end. Simple. Never really put much more thought into it. Little did I know the absorbency rate was down to an actual science. According to this side panel, if you bleed 6-9 grams of blood, you use the Regular. 9-12 grams, the Supers. A whopping 12-15 grams, the Super Plus-ers. I would like to know how the heck I am supposed to measure my flow?! How does one know just how many grams of blood one is shedding at any given time?! And just how did they do this test in the first place? Inquiring minds want to know. Or maybe not. It sounds like a bloody mess to me.