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.
This is something people from countries where people drink primarily hard liquor (Russia, Finland, much of Eastern Europe) know pretty well even if they don't know how to explain it. Another trick is to watch _what_ you eat. Meat and fat stay in the stomach for much longer, so if you focus on that, alcohol won't hit your liver all at once.