And it can't ever be accurate. Thing is, how drunk you're going to get within some time frame depends on what (and how much) you're eating, and whether your stomach had some food in it before you started drinking. If there's stuff in your stomach, its sphincter is closed while it's digesting it. Stomach itself absorbs alcohol (and nutrients) much slower than the intestine.
TL;DR: don't rely on this calculator to determine if you're too hammered to drive. If there's any doubt whatsoever, call an Uber or use public transportation.
