Whist I like the idea of a calculator the example you showed is very odd. So a couple sharing a room only slightly bigger pay almost double the rent of a single person. What is the point of them sharing the room? they might as well have separate rooms.
Also I'm not sure what renting situations are like all around the world but it also depends on the responsibility of the flatmates. For example if someone decides to up and leave with no goodbye who has to pay for the rent? If the house gets damaged with no one to blame who has to pay? the list of these questions goes on.
There is never a good solution to this problem and that is why it is usually a case by case basis. I personally like the idea of one person choosing what a room is worth and then if you want to live there then you pay that amount.
Yea, interestingly the two calculators give different answers for the same situation:
In both cases, the calculator puts great weight on the amount of shared space, where all four of the roommates pay equally. If I repeat the example posted here while specifying 'huge' shared space, then the cost-per person becomes even closer:
And to your point about one person being in charge and deciding, this is ideal, but often requires some level of financial stability for the primary renter, which might not be the case for new kids starting college.