What you are actually getting
A hex value that sits roughly where the CMYK build would, if the press and the paper were ideal. They are not. Treat it as a starting point for a screen mockup of something that will be printed, and expect the printed piece to look different.
The direction that disappoints
Going from print to screen is the friendlier direction: almost every CMYK build has an RGB equivalent, because the screen gamut is wider in the places that matter. The reverse is where colours fall off the edge.
So if a brand book gives you CMYK and you need a website colour, this gets you close. If the brand book also gives a hex or a Pantone, use that instead — it was chosen deliberately, and this is derived.
Total ink
The four numbers adding up over about 300% is a warning sign in real print work — too much ink for the paper to take. It is not something this page checks, because that limit belongs to the printer, not to the colour.
Questions
Will the screen colour match the print?+
No. It sits roughly where the CMYK build would under ideal conditions, which do not exist. Use it for a mockup and expect the printed piece to differ.
My brand book has CMYK and hex. Which do I use?+
The hex. It was chosen deliberately for screen; anything derived from the CMYK is a calculation, not a decision.
Does it check total ink coverage?+
No. The 300% limit that matters in real print work belongs to the printer and the paper, not to the colour, so this page does not pretend to enforce it.